Android: Enable ProGuard obfuscation

Reduces Chrome.apk by 65kb, Monochrome by 69kb.
Reduces uncompressed classes.dex 860kb / 895kb.
Reduces monochrome odex size on Android O by 936kb.

Tested locally that pages load (maps and yahoo).
Tests have been running with obfuscation enabled for some
time.

Bug: 620323
Change-Id: Ia2e939968670e6cc427db56469b3020e5ba8b266
Reviewed-on: https://chromium-review.googlesource.com/969120Reviewed-by: John Budorick <jbudorick@chromium.org>
Commit-Queue: agrieve <agrieve@chromium.org>
Cr-Commit-Position: refs/heads/master@{#544254}

chrome/android/chrome_public_apk_tmpl.gni

@@ -11,7 +11,7 @@ import("channel.gni")
 
 declare_args() {
   # Enables ProGuard obfuscation of Chromium packages.
-  enable_proguard_obfuscation = false
+  enable_proguard_obfuscation = true
 
   # Enable multidex in release builds.
   multidex_in_release = false