Android: Proguard rules to allow shrinking for <native>

base/android/proguard/chromium_code.flags

 # Copyright 2016 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.
 
 # Contains flags that can be safely shared with Cronet, and thus would be
 # appropriate for third-party apps to include.
 
 # Keep all annotation related attributes that can affect runtime
 -keepattributes RuntimeVisible*Annotations
 -keepattributes AnnotationDefault
 
 # Keep the annotations, because if we don't, the ProGuard rules that use them
 # will not be respected. These classes then show up in our final dex, which we
 # do not want - see crbug.com/628226.
 -keep @interface org.chromium.base.annotations.AccessedByNative
 -keep @interface org.chromium.base.annotations.CalledByNative
 -keep @interface org.chromium.base.annotations.CalledByNativeUnchecked
 -keep @interface org.chromium.base.annotations.RemovableInRelease
 -keep @interface org.chromium.base.annotations.UsedByReflection
 
 # Keeps for class level annotations.
 -keep @org.chromium.base.annotations.UsedByReflection class *
 
 # Keeps for method level annotations.
 -keepclasseswithmembers class * {
   @org.chromium.base.annotations.AccessedByNative <fields>;
 }
+# -includedescriptorclasses tells it to also keep return types and parameter
+# types.
 -keepclasseswithmembers,includedescriptorclasses class * {
   @org.chromium.base.annotations.CalledByNative <methods>;
 }
 -keepclasseswithmembers,includedescriptorclasses class * {
   @org.chromium.base.annotations.CalledByNativeUnchecked <methods>;
 }
 -keepclasseswithmembers class * {
   @org.chromium.base.annotations.UsedByReflection <methods>;
 }
 -keepclasseswithmembers class * {
   @org.chromium.base.annotations.UsedByReflection <fields>;
 }
--keepclasseswithmembers,includedescriptorclasses class * {
+# -keepclasseswithmembernames rather than -keepclasseswithmembers to allow
+# shrinking if it contains native methods but no @CalledByNative.
+-keepclasseswithmembernames,includedescriptorclasses class * {
   native <methods>;
 }
 
 # Remove methods annotated with this if their return value is unused.
 -assumenosideeffects class ** {
   @org.chromium.base.annotations.RemovableInRelease <methods>;
 }
 
 # Keep all CREATOR fields within Parcelable that are kept.
 -keepclassmembers class org.chromium.** implements android.os.Parcelable {
   public static *** CREATOR;
 }
 
 # Don't obfuscate Parcelables as they might be marshalled outside Chrome.
 # If we annotated all Parcelables that get put into Bundles other than
 # for saveInstanceState (e.g. PendingIntents), then we could actually keep the
 # names of just those ones. For now, we'll just keep them all.
 -keepnames class org.chromium.** implements android.os.Parcelable
 
 # Keep all enum values and valueOf methods. See
 # http://proguard.sourceforge.net/index.html#manual/examples.html
 # for the reason for this. Also, see http://crbug.com/248037.
 -keepclassmembers enum org.chromium.** {
     public static **[] values();
 }