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This CL adds resizeMode support for MediaStreamTrack.getConstraints()
and marks resizeMode as a supported constrainable property.

Support for getUserMedia(), applyConstraints() and getCapabilities()
will be added in follow-up CLs in this series.

Intent to Ship:
https://groups.google.com/a/chromium.org/forum/#!topic/blink-dev/V2srjdzRCXE

Bug: 854980
Change-Id: Ia23b8119768ab67ab498f0f8e2267aeb388f9fc3
Reviewed-on: https://chromium-review.googlesource.com/c/1309749Reviewed-by: Henrik Boström <hbos@chromium.org>
Reviewed-by: Philip Jägenstedt <foolip@chromium.org>
Commit-Queue: Guido Urdaneta <guidou@chromium.org>
Cr-Commit-Position: refs/heads/master@{#606856}