android_build_instructions.md: is_java_debug & enabled_incremental_javac

Change-Id: Icafce9c1651fc590cb21020b41e0d59f8ea0eba3
Reviewed-on: https://chromium-review.googlesource.com/c/1203392
Commit-Queue: agrieve <agrieve@chromium.org>
Reviewed-by: Eric Stevenson <estevenson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#600522}

docs/android_build_instructions.md

@@ -326,11 +326,23 @@ For information on running tests, see [Android Test Instructions](android_test_i
 
 ### Faster Edit/Deploy
 
+#### GN Args
+Args that affect build speed:
+ * `is_component_build = true` *(default=`is_debug`)*
+   * What it does: Uses multiple `.so` files instead of just one (faster links)
+ * `is_java_debug = true` *(default=`is_debug`)*
+   * What it does: Disables ProGuard (slow build step)
+ * `enable_incremental_javac = true` *(default=`false`)*
+   * What it does: Tries to compile only a subset of `.java` files within an
+     `android_library` for subsequent builds.
+   * Can cause infrequent (once a month-ish) failures due to not recompiling a
+     class that should be recompiled.
+
+#### Incremental Install
 "Incremental install" uses reflection and side-loading to speed up the edit
 & deploy cycle (normally < 10 seconds). The initial launch of the apk will be
 a little slower since updated dex files are installed manually.
 
-*   Make sure to set` is_component_build = true `in your GN args
 *   All apk targets have \*`_incremental` targets defined (e.g.
     `chrome_public_apk_incremental`) except for Webview and Monochrome