Fixing WebView ProGuard config for annotations

WebView used to not -keep its annotations. Thus, when trying to do
anything useful with ProGuard, it appeared that the annotations were not
being obeyed, since ProGuard had already stripped them out.

Note - since we still have the -keepnames class *** { *; }, this effectively
prevents any real optimizations from being run.

This change brings the .dex size down 7356 B, and removes 12 types, 5 fields,
and 57 strings. However, these all seem inconsequential, as this is the
method/class diff of this change:

org.chromium.base.Log
-  void d(java.lang.String,java.lang.String,java.lang.Object)
-  void v(java.lang.String,java.lang.String)
-  void d(java.lang.String,java.lang.String,java.lang.Object,java.lang.Object,java.lang.Object)
-  java.lang.String formatLogWithStack(java.lang.String,java.lang.Object[])
-  void d(java.lang.String,java.lang.String)
-  void v(java.lang.String,java.lang.String,java.lang.Object,java.lang.Object,java.lang.Object)
-  void <init>()
-  void d(java.lang.String,java.lang.String,java.lang.Object,java.lang.Object)
-  java.lang.String getCallOrigin()
-  void debug(java.lang.String,java.lang.String,java.lang.Object[])
-  void verbose(java.lang.String,java.lang.String,java.lang.Object[])

BUG=541543,583143,624827,627139

Review-Url: https://codereview.chromium.org/2147743002
Cr-Commit-Position: refs/heads/master@{#405491}

android_webview/apk/java/proguard.flags

@@ -8,10 +8,14 @@
 
 # Keep all runtime visible annotations.
 -keepattributes RuntimeVisibleAnnotations
-
-# Disable optimization as this causes proguard to remove seemingly random stuff,
-# including things explicitly kept in this configuration.
--dontoptimize
+# Keeping annotations - if we remove these keeps, ProGuard will eliminate the
+# annotations themselves, which will prevent them from affecting ProGuard's
+# optimization run.
+-keep @interface **.AccessedByNative
+-keep @interface **.CalledByNative
+-keep @interface **.CalledByNativeUnchecked
+-keep @interface **.RemovableInRelease
+-keep @interface **.UsedByReflection
 
 # Keep the factory and its public members; it's the main entry point used by the
 # framework.
@@ -39,11 +43,19 @@
   native <methods>;
 }
 
-# Keep things explicitly marked as used by reflection
+# Keep classes marked as used by reflection
+-keep @**.UsedByReflection class com.android.webview.chromium.**,org.chromium.**
+
+# Keep members explicitly marked as used by reflection
 -keepclasseswithmembers class com.android.webview.chromium.**,org.chromium.** {
   @**.UsedByReflection *;
 }
 
+# Remove methods annotated with this if their return value is unused
+-assumenosideeffects class ** {
+  @org.chromium.base.annotations.RemovableInRelease <methods>;
+}
+
 # Linker dynamically casts to $TestRunner when running tests. We don't run these
 # tests in WebView.
 -dontnote org.chromium.base.library_loader.Linker$TestRunner