Android: Better -keep rules for BackgroundTasks

As suggested by r8 team.

Bug: 985372
Change-Id: I2ca93568d06800a7bf82476ef0ca106492084fb1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1711251Reviewed-by: Tommy Nyquist <nyquist@chromium.org>
Commit-Queue: Andrew Grieve <agrieve@chromium.org>
Cr-Commit-Position: refs/heads/master@{#679977}

chrome/android/java/monochrome_public_bundle.proguard_flags.expected

@@ -358,18 +358,9 @@
 # found in the LICENSE file.
 
 # Classes are created via reflection and persisted across app updates, so their
-# class names must not be obfuscated.
--keepnames class ** implements org.chromium.components.background_task_scheduler.BackgroundTask {}
-
-# Also need to keep their default constructors.
--keepclassmembers class ** implements org.chromium.components.background_task_scheduler.BackgroundTask {
-  public <init>();
-}
-
-# TODO(agrieve): Delete this rule once r8 is fixed to not discard
-# GCMBackgroundTask.<init>() with only the above rules.
-# https://issuetracker.google.com/issues/137890409
--keep class ** implements org.chromium.components.background_task_scheduler.BackgroundTask {
+# class names must not be obfuscated, and constructors kept.
+-if class ** implements org.chromium.components.background_task_scheduler.BackgroundTask
+-keep class <1> {
   public <init>();
 }
 

components/background_task_scheduler/android/proguard.flags

@@ -3,17 +3,8 @@
 # found in the LICENSE file.
 
 # Classes are created via reflection and persisted across app updates, so their
-# class names must not be obfuscated.
--keepnames class ** implements org.chromium.components.background_task_scheduler.BackgroundTask {}
-
-# Also need to keep their default constructors.
--keepclassmembers class ** implements org.chromium.components.background_task_scheduler.BackgroundTask {
-  public <init>();
-}
-
-# TODO(agrieve): Delete this rule once r8 is fixed to not discard
-# GCMBackgroundTask.<init>() with only the above rules.
-# https://issuetracker.google.com/issues/137890409
--keep class ** implements org.chromium.components.background_task_scheduler.BackgroundTask {
+# class names must not be obfuscated, and constructors kept.
+-if class ** implements org.chromium.components.background_task_scheduler.BackgroundTask
+-keep class <1> {
   public <init>();
 }