Android: Add proguard flag to prevent obfuscation of BackgroundTask classes

These classes need to be the same across releases.

Bug: 913116
Change-Id: I57dc8928f24944f0234863f5fcfc248886c6a7cc
Reviewed-on: https://chromium-review.googlesource.com/c/1371244Reviewed-by: Filip Gorski <fgorski@chromium.org>
Commit-Queue: agrieve <agrieve@chromium.org>
Cr-Commit-Position: refs/heads/master@{#615519}

components/background_task_scheduler/BUILD.gn

@@ -47,6 +47,8 @@ if (is_android) {
       "android/java/src/org/chromium/components/background_task_scheduler/TaskParameters.java",
     ]
 
+    proguard_configs = [ "android/proguard.flags" ]
+
     deps = [
       ":background_task_scheduler_task_ids_java",
       "$google_play_services_package:google_play_services_base_java",

components/background_task_scheduler/android/proguard.flags

+# Copyright 2018 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.
+
+# Classes are created via reflection, so may not be renamed.
+-keepnames class * implements org.chromium.components.background_task_scheduler.BackgroundTask {
+  public <init>();
+}