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This is the first CL in adding getDisplayMedia() support in chromium. See the
bug and linked design doc for the detailed plan.

This CL adds getDisplayMedia() interface behind runtime enabled features flag
"GetDisplayMedia". This call will use the same call stack as getUserMedia() but
have different media request type and handling of constraints.
- Adds WebUserMediaRequest enums to differentiate this call.
- Adds specific handling of constraints in UserMediaRequest::Create(). As
defined in the spec, this accepts empty constraints.
- Currently ends up in a NotSupported error as chromium changes are landing.
- Adds first set of Layout Tests to check these expectations.

Intent To Implement:
https://groups.google.com/a/chromium.org/forum/#!msg/Blink-dev/j7k2nI_9nng/OE6IvgJyAQAJ

Bug: 326740
Change-Id: I03c54cab259a57db99449d9c4510c484f22c5b93
Reviewed-on: https://chromium-review.googlesource.com/1155942Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Guido Urdaneta <guidou@chromium.org>
Commit-Queue: Emircan Uysaler <emircan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#580733}