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This should make the chromium.webrtc waterfall work like the old
test config in recipes. It incorporates some fixes made in the
recipe code and creates composite test suites for the chromium.webrtc
bots.

Bug: 877018
Change-Id: I11b91c4e9516844086978b5a63b816576197585b
Reviewed-on: https://chromium-review.googlesource.com/c/1280669Reviewed-by: Kenneth Russell <kbr@chromium.org>
Commit-Queue: Patrik Höglund <phoglund@chromium.org>
Cr-Commit-Position: refs/heads/master@{#599947}