Fixing multidex on L+ with R8

Previously, R8 would fail if it went over the dex limit on an L+ target (like
Monochrome). However, since R8 reduced the methods pretty dramatically, it
wasn't being seen.

Bug: 908988
Change-Id: Ic393d217f8f5f493db505136e3f4c0374d98c44a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1594203
Commit-Queue: Andrew Grieve <agrieve@chromium.org>
Reviewed-by: Andrew Grieve <agrieve@chromium.org>
Reviewed-by: Eric Stevenson <estevenson@chromium.org>
Auto-Submit: Sam Maier <smaier@chromium.org>
Cr-Commit-Position: refs/heads/master@{#658236}

build/android/arbitrary_main_dex.flags

+# Copyright 2019 The Chromium Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+# Proguard flags to pass to R8 which do nothing, and thus allows R8 to
+# arbitrarily assign code to different dex files. If we pass a flags file with
+# no rules, R8 applies default main dex rules which usually include too much
+# and pushes us over the main dex limit. We can't pass higher --min-api since we
+# also want to allow multidex in libraries, which are compiled with a low
+# min-api. Thus, we have rules which won't do anything to pass as a
+# main-dex-rule.
+
+-keep class this.should.not.match.anything {
+  *;
+}

build/config/android/internal_rules.gni

@@ -1300,9 +1300,11 @@ if (enable_java_templates) {
             ]
             inputs += [ _main_dex_rules ]
           } else {
-            if (defined(invoker.extra_main_dex_proguard_config)) {
-              not_needed(invoker, [ "extra_main_dex_proguard_config" ])
-            }
+            args += [
+              "--main-dex-rules-path",
+              rebase_path("//build/android/arbitrary_main_dex.flags",
+                          root_build_dir),
+            ]
           }
         }