Disable proguard optimization that removes write-only fields.

base/android/proguard/chromium_apk.flags

 # 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.
 
 # Contains flags that we'd like all Chromium .apks to use.
 
 # Keep line number information, useful for stack traces.
 -keepattributes SourceFile,LineNumberTable
 
 # Keep all CREATOR fields within Parcelable that are kept.
@@ skipping 29 matching lines @@
 # -----------------------------------------------------------------
 #          1          | 0:48 |  5805676  |         488972
 #          2          | 1:07 |  5777376  |         487092
 #          3          | 1:24 |  5772192  |         486596
 #          4          | 1:42 |  5771124  |         486484
 #          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/*
+# Write-only field removal removes some static fields that are intended to stay
+# around until the end of execution.
+-optimizations !class/merging/*,!field/removal/writeonly
 
 # Allowing Proguard to change modifiers. This change shrinks the .dex size by
 # ~1%, and reduces the method count by ~4%.
 -allowaccessmodification
 
 # The support library contains references to newer platform versions.
 # Don't warn about those in case this app is linking against an older
 # platform version.  We know about them, and they are safe.
 -dontwarn android.support.**