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Makes the arg take effect more universally by having it be a flag on
proguard.py rather than work by having targets add the correct proguard
configs.

Also enabled -repackageclasses '' for L8, which means it should
produce a smaller .dex file.

Bug: 1168672
Change-Id: I9c07d337755efedbdf1fabbe8822a9fdfba49fa6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2641306
Commit-Queue: David Trainor <dtrainor@chromium.org>
Commit-Queue: Andrew Grieve <agrieve@chromium.org>
Reviewed-by: Sam Maier <smaier@chromium.org>
Cr-Commit-Position: refs/heads/master@{#845894}