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Currently all mojom targets are generated on Android for Java which
then causes them to all be compiled as the bots compile all targets.

This makes generating Java bindings for Android explicit, and adds the
flag to each mojom target that is reachable during `gn gen`.

The flag is overridden by cpp_only, which would prevent Java bindings
from being generated.

This saves about 2 minutes per parallel compilation task when building
the .javac.jar step, which is a dominant part of compile. In the
experiment the total time spend on this step went from 1h21m to 1h10m.

The .jar step also improved from 1h1m to 53m, with weighted time from
6m6s to 5m4s.

R=agrieve@chromium.org, rockot@chromium.org
TBR=jam

Bug: 603143
Change-Id: I4c636ce051abcd7a89efc46224b2405cef8110dd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1868986
Commit-Queue: danakj <danakj@chromium.org>
Reviewed-by: Ken Rockot <rockot@google.com>
Reviewed-by: Andrew Grieve <agrieve@chromium.org>
Cr-Commit-Position: refs/heads/master@{#708315}