Use keepclassmembers for android.webkit.JavascriptInterface in proguard

This CL uses -keepclassmembers for
android.webkit.JavascriptInterface instead of
-keepclasseswithmembers in proguard.

The reason is because keeping methods is enough for files
annotated with for android.webkit.JavascriptInterface.

BUG=634998

Review-Url: https://codereview.chromium.org/2250963004
Cr-Commit-Position: refs/heads/master@{#412818}

base/android/base_proguard_config.flags

@@ -18,7 +18,7 @@
 -keep @org.chromium.base.annotations.UsedByReflection class *
 
 # Keeps for method level annotations.
--keepclasseswithmembers class * {
+-keepclassmembers class * {
   @android.webkit.JavascriptInterface <methods>;
 }
 -keepclasseswithmembers class * {