Rename building_dex.md -> java_toolchain.md

And add step for ErrorProne.

Change-Id: Ic6054026e5c8d7b47a0f212499076b8f8d5ff8dd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1855470
Reviewed-by: Tibor Goldschwendt <tiborg@chromium.org>
Commit-Queue: Andrew Grieve <agrieve@chromium.org>
Cr-Commit-Position: refs/heads/master@{#705118}

build/android/docs/README.md

@@ -2,8 +2,8 @@
 
 * [android_app_bundles.md](android_app_bundles.md)
 * [build_config.md](build_config.md)
-* [building_dex.md](building_dex.md)
 * [coverage.md](coverage.md)
+* [java_toolchain.md](java_toolchain.md)
 * [lint.md](lint.md)
 * [life_of_a_resource.md](life_of_a_resource.md)
 * [../incremental_install/README.md](../incremental_install/README.md)

build/android/docs/building_dex.md → build/android/docs/java_toolchain.md

@@ -20,7 +20,7 @@ also have a default `jar_excluded_patterns` set (more on that later):
 All targets names must end with "_java" so that the build system can distinguish
 them from non-java targets (or [other variations](https://cs.chromium.org/chromium/src/build/config/android/internal_rules.gni?rcl=ec2c17d7b4e424e060c3c7972842af87343526a1&l=20)).
 
-## Step 1: Compile
+## Step 1a: Compile with javac
 
 This step is the only step that does not apply to prebuilt targets.
 
@@ -39,6 +39,18 @@ This step is the only step that does not apply to prebuilt targets.
     recompiled.
   * Prefer smaller targets to avoid slow compiles.
 
+## Step 1b: Compile with ErrorProne
+
+This step can be disabled via GN arg: `use_errorprone_java_compiler = false`
+
+* Concurrently with step 1a: [ErrorProne] compiles java files and checks for bug
+  patterns, including some [custom to Chromium][ep_plugins].
+* ErrorProne used to replace step 1a, but was changed to a concurrent step after
+  being identified as being slower.
+
+[ErrorProne]: https://errorprone.info/
+[ep_plugins]: /tools/android/errorprone_plugin/
+
 ## Step 2: Creating an .interface.jar
 
 This step happens in parallel with subsequent steps.