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This CL pulls in base's proguard flags when compiling
cronet_sample_apk and cronet_package.

Cronet will not need to duplicate proguard flags and can
benefit from base proguard file changes.

This saves about 1882 - 1471 = 411 methods from our dex
count for CronetSample.apk and 10kB(4.35%).

BUG=634998

Review-Url: https://codereview.chromium.org/2214013002
Cr-Commit-Position: refs/heads/master@{#417060}