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Update proguard.cfg to reflect that @UsedByReflection was moved to base/.
Add --release option to cr_cronet.py tool.

Notes:
- Chromium debug build does not apply proguard at all.
- Chromium instrumentation test does not allow custom proguard config (but it uses proguard config from instrumented app, which is nice).

BUG=390267

Review URL: https://codereview.chromium.org/512953002

Cr-Commit-Position: refs/heads/master@{#294671}