Android: Enable vertical class merging ProGuard optimization

Shrinks monochrome_apk dex by 53kb uncompressed, 17kb compressed.
Shrinks method_count by 1400.

Bug: 620323
Change-Id: Ie9f7c37a20895ecd1d3f1f17357a663252a1be36
Reviewed-on: https://chromium-review.googlesource.com/981052Reviewed-by: Eric Stevenson <estevenson@chromium.org>
Commit-Queue: agrieve <agrieve@chromium.org>
Cr-Commit-Position: refs/heads/master@{#545857}

base/android/proguard/chromium_apk.flags

@@ -48,9 +48,8 @@
 #          5          | 1:56 |  5770504  |         486432
 -optimizationpasses 3
 
-# Class merging messes up stacktraces beyond the point of them being
-# deobfuscatable. If turned on, it would give us a 2% reduction in .dex size.
--optimizations !class/merging/*
+# Horizontal class merging marginally increases dex size (as of Mar 2018).
+-optimizations !class/merging/horizontal
 
 # Allowing Proguard to change modifiers. This change shrinks the .dex size by
 # ~1%, and reduces the method count by ~4%.