Enable video requests from getDisplayMedia()

This CL allows getDisplayMedia() calls where video is enabled to cross from
blink to content side. Additionally, it applies the constraints handling
mentioned in the spec(current state) and adds Layout tests.

Bug: 326740
Change-Id: I864f485595c63485cc70e519796072f928e22e4c
Reviewed-on: https://chromium-review.googlesource.com/1179310
Reviewed-by: Harald Alvestrand <hta@chromium.org>
Reviewed-by: Guido Urdaneta <guidou@chromium.org>
Commit-Queue: Emircan Uysaler <emircan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#585099}
6 files changed