Disable proguard optimization that removes write-only fields.

base/android/proguard/chromium_apk.flags

Index: base/android/proguard/chromium_apk.flags
diff --git a/base/android/proguard/chromium_apk.flags b/base/android/proguard/chromium_apk.flags
index b5ffb7f3013c01b5c9c5acd203350c867e08c4c5..4bdfad9c34a9d73322487dcead7ef2badb3936fb 100644
--- a/base/android/proguard/chromium_apk.flags
+++ b/base/android/proguard/chromium_apk.flags
@@ -47,7 +47,9 @@
 
 # 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%.