USB/WebUSB documentation, adding README.md files in several locations.

Bug: 740070
Change-Id: I9d0a7371c22856a641ad88910d55ba844aca4191
Reviewed-on: https://chromium-review.googlesource.com/572332
Commit-Queue: Vincent Scheib <scheib@chromium.org>
Reviewed-by: Reilly Grant <reillyg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#488481}

device/usb/README.md

@@ -0,0 +1,53 @@
+# USB
+
+`device/usb` abstracts [Universal Serial Bus](https://en.wikipedia.org/wiki/USB)
+concepts across multiple platforms.
+
+Clients should use the [public Mojo interface](/device/usb/public/interfaces).
+
+
+## USB ID Repository
+
+`/third_party/usb_ids/usb.ids` is imported regularly to provide human-readable
+descriptions of USB devices.
+
+
+## Ongoing Work transitioning away from `libusb`
+
+`UsbService` is implemented by `UsbServiceImpl` based on `third_party/libusb`.
+
+Work is ongoing to implement each platform directly, without using `libusb`.
+
+*   `UsbServiceAndroid` done.
+*   `UsbServiceLinux` done.
+*   macOS not started.
+*   `UsbServiceWin` in progress. Enable via `chrome://flags/#new-usb-backend`
+
+
+## Testing
+
+### Unit Tests
+
+Standard use of `*_unittest.cc` files for must code coverage.
+
+
+### Fuzzers
+
+[libFuzzer] tests are in `*_fuzzer.cc` files. They test for bad input from
+devices, e.g. when parsing device descriptors during device enumeration.
+
+[libFuzzer]: /testing/libfuzzer/README.md
+
+
+### Gadget Tests
+
+[USB/HID API Testing with Gadgets] describes a framework for testing the Chrome
+USB, HID and serial device APIs with real devices on generally accessible
+hardware.
+
+[USB/HID API Testing with Gadgets]: https://docs.google.com/document/d/1O9jTlOAyeCwZX_XRbmQmNFidcJo8QZQSaodP-wmyess
+
+Unit tests using the gadget can be run manually with a hardware "gadget". These
+unit tests all call [UsbTestGadget::Claim].
+
+[UsbTestGadget::Claim]: https://cs.chromium.org/search/?q=UsbTestGadget::Claim&type=cs

device/usb/mojo/device_manager_impl.h

@@ -26,8 +26,8 @@ namespace usb {
 
 class PermissionProvider;
 
-// Implementation of the public DeviceManager interface. This interface can be
-// requested from the devices app located at "devices", if available.
+// Implements the public Mojo UsbDeviceManager interface by wrapping the
+// UsbService instance.
 class DeviceManagerImpl : public mojom::UsbDeviceManager,
                           public UsbService::Observer {
  public:

third_party/WebKit/LayoutTests/external/wpt/resources/chromium/README.md

@@ -1,4 +1,4 @@
 This directory contains Chromium-specific test resources.
 
-The files `mojo_bindings.js` and `*.mojom.js` are automatically generated by the
-Chromium build process and should not be edited manually.
+The files `mojo_bindings.js` and `*.mojom.js` are manually copied from the
+Chromium build process's generated files and should not be edited manually.

third_party/WebKit/LayoutTests/external/wpt/webusb/README.md

@@ -0,0 +1,12 @@
+# WebUSB Testing
+
+WebUSB testing relies on the [WebUSB Testing API] which must be
+provided by browsers under test.
+
+In this test suite `resources/usb-helpers.js` detects and triggers
+the API to be loaded as needed.
+
+The Chromium implementation is provided by
+`../resources/chromium/webusb-test.js`.
+
+[WebUSB Testing API]: https://wicg.github.io/webusb/test/

third_party/WebKit/Source/modules/webusb/README.md

@@ -0,0 +1,17 @@
+# WebUSB Blink Module
+
+`Source/modules/webusb` implements the renderer process details and bindings
+for the [WebUSB specification]. It communicates with the browser process through the [public Mojo interface] of `//device/usb` to the [UsbService].
+
+[WebUSB specification]: https://wicg.github.io/webusb/
+[public Mojo interface]: /device/usb/public/interfaces
+[UsbService]: /device/usb/usb_service.h
+
+
+## Testing
+
+WebUSB is primarily tested in [Web Platform Tests].
+Chromium implementation details are tested in [Layout Tests].
+
+[Web Platform Tests]: ../../../LayoutTests/external/wpt/webusb/
+[Layout Tests]: ../../../LayoutTests/usb/