Add GN arg "enable_proguard_obfuscation"

chrome/android/java/proguard.flags

-# As of August 11, 2016, obfuscation was tested and passed on the trybots,
-# saving us 660kb on our .dex size and 53kb memory/process. Enable obfuscation
-# as soon as Crash supports ReTrace's deobfuscation of stack traces.
-#
-# To enable obfuscation, remove the -keepnames rule for
-# com.google.android.apps.chrome.** and org.chromium.**, and uncomment the
-# following two lines:
-# -renamesourcefileattribute PG
-# -repackageclasses ""
-
-# Disable obfuscation for the following two packages.
--keepnames,allowoptimization class com.google.android.apps.chrome.**,org.chromium.** {
-  *;
-}
+# 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.
 
 # Fragments are loaded using reflection via Fragment.instantiate(Context,String)
 # This can be improved upon - see crbug.com/622023.
 -keep public class com.google.android.apps.chrome** extends android.app.Fragment
 -keep public class org.chromium** extends android.app.Fragment
 
 # These classes aren't themselves referenced, but __ProcessService[0,1,2...] are
 # referenced, and we look up these services by appending a number onto the name
 # of the base class. Thus, we need to keep the base class name around so that
 # the child classes can be looked up.
 -keep class org.chromium.content.app.SandboxedProcessService
 -keep class org.chromium.content.app.PrivilegedProcessService
 
 # TODO(ianwen): remove this config once proguard file inclusion from AAR is supported.
 # CoordinatorLayout resolves the behaviors of its child components with reflection.
 -keep public class * extends android.support.design.widget.CoordinatorLayout$Behavior {
     public <init>(android.content.Context, android.util.AttributeSet);
     public <init>();
 }
 
 # SearchView is used in website_preferences_menu.xml and is constructed by
 # Android using reflection.
 -keep class android.support.v7.widget.SearchView {
   public <init>(...);
 }
 
 # Google Play Services warnings are about its resources.
 -dontwarn com.google.android.gms.R**