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The id is necessary for single-process-mash so that the browser process can
know it's id and lookup windows (everything runs in the same process in
single-process-mash). Outside of single-process-mash the id is really only
useful for debugging.

BUG=756085
TEST=covered by tests

Change-Id: I3819c5492fe2bfeeb243c44c36cc948168824ab6
Reviewed-on: https://chromium-review.googlesource.com/c/1318447Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Commit-Queue: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#605867}