Android: Fix cronet proguard failure when enable_chrome_android_internal=true

Tested that all targets now build via:
autoninja -C out/Release $(build/android/list_java_targets.py --proguard-enabled)

Bug: 1155547
Change-Id: Ie32d62e108778c219a190e363be9c82b747a2a02
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2574597
Auto-Submit: Andrew Grieve <agrieve@chromium.org>
Commit-Queue: Paul Jensen <pauljensen@chromium.org>
Reviewed-by: Paul Jensen <pauljensen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#833824}

components/cronet/android/test/proguard.cfg

@@ -11,6 +11,9 @@
 # https://android.googlesource.com/platform/sdk/+/marshmallow-mr1-release/files/proguard-android.txt#54
 -dontwarn android.support.**
 
+# Temporary fix for https://bugs.chromium.org/p/chromium/issues/detail?id=1155547
+-dontwarn com.google.protobuf.**
+
 # Do not obfuscate this class for testing since some of the tests check the class
 # name in order to check that an instantiated engine is the Java one.
 -keepnames class org.chromium.net.impl.JavaCronetEngine