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This reverts commit 786379ee.

Reason for revert: Broke "generate_build_files" step for 
release builders.

Original change's description:
> [proguard] Include proguard info in sourcefile attribute.
> 
> Instead of setting the sourcefile to PG when obfuscating Java, set it
> to :chromium:{mapping file}:{channel}:{version code} so that
> 
> a) We can recognise chrome/webview stack frames by this string
> b) The information required to uniquely identify the proguard mapping
>    file in question is directly available in stack traces.
> 
> Bug: 1044585
> Change-Id: Ib4fabccfb7693e5e97edc6f29be1cd6cb61fa1e6
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2036091
> Commit-Queue: Tobias Sargeant <tobiasjs@chromium.org>
> Reviewed-by: Andrew Grieve <agrieve@chromium.org>
> Reviewed-by: Piotr Bialecki <bialpio@chromium.org>
> Reviewed-by: Richard Coles <torne@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#740625}

TBR=torne@chromium.org,agrieve@chromium.org,tobiasjs@chromium.org,bialpio@chromium.org

Change-Id: Iad5e489fbb13b1d9b89545bc648e94206e0c7c90
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 1044585
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2050022Reviewed-by: Brandon Wylie <wylieb@chromium.org>
Commit-Queue: Brandon Wylie <wylieb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#740750}