Mash cleanup: Remove accelerator_controller.mojom and volume.mojom

1. Prevent Chrome reaching directly into ash::AcceleratorController by
creating a pure virtual interface. The implementation is renamed
AcceleratorControllerImpl.

2. Move volume accelerator handling to ash. Convert VolumeController
interface into a single callback that Chrome provides. This callback
is run whenever the volume is changed via the keyboard.

Bug: 958225, 958131
Change-Id: Icc59608ad092eb9f53cda0637d0585ffdfa7f925
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1595507
Commit-Queue: Evan Stade <estade@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Auto-Submit: Evan Stade <estade@chromium.org>
Cr-Commit-Position: refs/heads/master@{#657774}

ash/BUILD.gn

@@ -28,7 +28,6 @@ component("ash") {
     "ash_service.h",
 
     # TODO: move the following to source. Do NOT add new files here.
-    "accelerators/accelerator_controller.h",
     "accessibility/accessibility_controller.h",
     "accessibility/accessibility_delegate.h",
     "accessibility/focus_ring_controller.h",
@@ -97,7 +96,8 @@ component("ash") {
     "accelerators/accelerator_commands.h",
     "accelerators/accelerator_confirmation_dialog.cc",
     "accelerators/accelerator_confirmation_dialog.h",
-    "accelerators/accelerator_controller.cc",
+    "accelerators/accelerator_controller_impl.cc",
+    "accelerators/accelerator_controller_impl.h",
     "accelerators/accelerator_ids.h",
     "accelerators/accelerator_table.cc",
     "accelerators/accelerator_table.h",