r8/README.chromium: Add filing bug instructions

Bug: None
Change-Id: I3473e5798ea1b790a22adecd033777637aca023b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2574829
Commit-Queue: Andrew Grieve <agrieve@chromium.org>
Reviewed-by: Sam Maier <smaier@chromium.org>
Cr-Commit-Position: refs/heads/master@{#833851}

build/android/gyp/proguard.py

@@ -118,6 +118,10 @@ def _ParseOptions():
   parser.add_argument('--show-desugar-default-interface-warnings',
                       action='store_true',
                       help='Enable desugaring warnings.')
+  parser.add_argument('--dump-inputs',
+                      action='store_true',
+                      help='Use when filing R8 bugs to capture inputs.'
+                      ' Stores inputs to r8inputs.zip')
   parser.add_argument(
       '--stamp',
       help='File to touch upon success. Mutually exclusive with --output-path')
@@ -238,6 +242,8 @@ def _OptimizeWithR8(options,
     ]
     if options.disable_outlining:
       cmd += ['-Dcom.android.tools.r8.disableOutlining=1']
+    if options.dump_inputs:
+      cmd += ['-Dcom.android.tools.r8.dumpinputtofile=r8inputs.zip']
     cmd += [
         '-cp',
         options.r8_path,

third_party/r8/README.chromium

@@ -63,3 +63,13 @@ java -cp third_party/r8/lib/r8.jar com.android.tools.r8.BackportedMethodList --m
 Update instructions for desugar_jdk_libs.json:
 * Find desugar.json as described above and copy to desugar_jdk_libs.json.
 * Remove all mentions of ConcurrentHashMap (and remove any trailing commas if necessary).
+
+How to file bugs against R8:
+* Copy & paste the failing ninja command (starts with proguard.py), and add --dump-inputs.
+* File bug at go/r8bug
+* Things to include (use discretion if some are not relevant):
+  * src revision bug reproduces at
+  * Repro steps (gn gen & autoninja commands)
+    * Prefer enable_chrome_android_internal=false
+  * The r8inputs.zip from --dump-inputs
+  * Any relevant dexdump analysis