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This reverts commit 987d1fa4.

Reason for revert: Broke internal targets depending on cronet, that treat proguard warnings as errors

Original change's description:
> Removing unnecessary dontwarns from cronet proguard configs
> 
> Bug: 887942
> Change-Id: I120d6aa7681d7d86ce3ee9d2b8add7bd5dc0e0da
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1804075
> Auto-Submit: Sam Maier <smaier@chromium.org>
> Commit-Queue: Ryan Hamilton <rch@chromium.org>
> Reviewed-by: Ryan Hamilton <rch@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#696870}

TBR=rch@chromium.org,smaier@chromium.org

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: 887942
Change-Id: I58746c8ad28176c6115b5b2ec205bd372f3a7513
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1812133Reviewed-by: Eric Roman <eroman@chromium.org>
Commit-Queue: Eric Roman <eroman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#697842}