Android: Mark vertical class merging as a broken optimization

This allows the optimization to be enabled when using custom
proguard versions. With our current version of ProGuard (5.2.1),
it can cause VerifyErrors at runtime (see PS1 trybot failures).

Note that when the pass is enabled, I'm seeing:

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

Bug: 620323, 825995
Change-Id: I08146a8a84d4f466ed91834728914aa116bca7a5
Reviewed-on: https://chromium-review.googlesource.com/986532Reviewed-by: John Budorick <jbudorick@chromium.org>
Reviewed-by: Eric Stevenson <estevenson@chromium.org>
Commit-Queue: agrieve <agrieve@chromium.org>
Cr-Commit-Position: refs/heads/master@{#550959}

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%.

build/android/gyp/proguard.py

@@ -13,6 +13,8 @@ from util import proguard_util
 
 
 _DANGEROUS_OPTIMIZATIONS = [
+    # See crbug.com/825995 (can cause VerifyErrors)
+    "class/merging/vertical",
     "class/unboxing/enum",
     # See crbug.com/625992
     "code/allocation/variable",