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This provides support for type="chrome_messages_json_gzip" in
grit/build.py, which compresses the output files (which should have
.gz appended to the name, e.g. messages.json.gz).

This makes it fairly simple to convert an extension (e.g. chromevox)
to use compressed l10n files.

Original CL:
https://chromium-review.googlesource.com/c/chromium/src/+/1913610

For changes reviewed in the original CL:
TBR=agrieve@chromium.org

Bug: 1023568
Change-Id: Ibf8dfa374f5e759b3b459d007fde2423f83873a5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1931097
Commit-Queue: Steven Bennetts <stevenjb@chromium.org>
Reviewed-by: David Tseng <dtseng@chromium.org>
Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#720686}