MonochromePublic.apk release builds are very close to the main dex limit
and updating proguard reduces the number of methods by about 3000.

6.0.3 cannot be used for all APKs yet due to a bug which causes
cronet_test_instrumentation_apk to fail.

The version mismatch should be temporary as we plan to switch to R8 in the
future (https://crbug.com/868770).

Bug: 857572
Cq-Include-Trybots: master.tryserver.chromium.android:android_cronet_tester
Change-Id: I3676a42908c5046dc3f3831e282150702038140d
Reviewed-on: https://chromium-review.googlesource.com/1154366Reviewed-by: Yaron Friedman <yfriedman@chromium.org>
Commit-Queue: Eric Stevenson <estevenson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#579250}