[Background Sync] Use GcmNetworkManager to start the browser for sync events.

chrome/android/java/proguard.flags

 # Keep line number information, useful for stack traces.
 -keepattributes SourceFile,LineNumberTable
 
 # Keep the annotations.
 -keep @interface ***
 
 # Disable obfuscation for the following two packages.
 -keepnames class com.google.android.apps.chrome.**,org.chromium.** {
   *;
 }
@@ skipping 95 matching lines @@
 }
 
 -keep class android.support.v7.app.AlertDialog** {
   *;
 }
 
 -keep class com.google.android.gms.cast.CastMediaControlIntent* {
   *;
 }
 
+-keepnames class com.google.android.gms.gcm.** {
+  *;
+}
+
 -keepnames class jp.tomorrowkey.android.gifplayer.** {
   public *;
 }
 
 # Used in tests.
 -keep class android.support.v4.view.ViewCompat {
   public static int getLayoutDirection(android.view.View);
 }
 
 # flingViewport is used by Android WebView and a Chrome test.
@@ skipping 11 matching lines @@
 # Needed to compile ChromeTest.apk
 -keep class android.support.customtabs.ICustomTabsCallback** {
   *;
 }
 
 #-keep public class android.net.http.SslCertificate
 #-keep public class android.webkit.WebVie
 
 #-dontwarn android.webkit.WebView
 -dontwarn android.net.http.SslCertificate