Update component build documentation.

This build was recently made the default, and more documentation was added. This brings everything in sync.

Review-Url: https://codereview.chromium.org/2210813002
Cr-Commit-Position: refs/heads/master@{#409863}

build/config/BUILDCONFIG.gn

@@ -144,6 +144,9 @@ declare_args() {
   # Component build. Setting to true compiles targets declared as "components"
   # as shared libraries loaded dynamically. This speeds up development time.
   # When false, components will be linked statically.
+  #
+  # For more information see
+  # https://chromium.googlesource.com/chromium/src/+/master/docs/component_build.md
   is_component_build = is_debug && current_os != "ios"
 }
 

docs/component_build.md

@@ -5,17 +5,21 @@
 Release builds are “static” builds which compile to one executable and
 zero-to-two shared libraries (depending on the platform). This is efficient at
 runtime, but can take a long time to link because so much code goes into a
-single binary. When you set the GN build variable
+single binary.
+
+In a component build, many smaller shared libraries will be generated. This
+speeds up link times, and means that many changes only require that the local
+shared library be linked rather than the full executable, but at the expense of
+program load-time performance.
+
+The component build is currently the default for debug non-iOS builds (it
+doesn’t work for iOS). You can force it on for release builds using the
+[GN build arg](https://www.chromium.org/developers/gn-build-configuration):
 
 ```python
 is_component_build = true
 ```
 
-the build will generate many smaller shared libraries. This speeds up link
-times, and means that many changes only require that the local shared library
-be linked rather than the full executable, but at the expense of program
-load-time performance.
-
 ### How to make a component
 
 Defining a component just means using the GN “component” template instead