Media Capture Depth Stream Extensions API: videoKind settings and constraint.

This enables selection of depth stream on Linux; both color and
depth device have the same Label and it is not possible to apart
depth from color stream (https://crbug.com/686756).

Under experimental MediaCaptureDepth feature, implements [1]:

partial dictionary MediaTrackConstraintSet {
    ConstrainDOMString videoKind;
};

and constraining logic applying it to MediaStreamTrack object.

partial dictionary MediaTrackSettings {
    DOMString videoKind;
};

This patch is verified to work with fake capture device and RealSense SR300.

[1]
https://w3c.github.io/mediacapture-depth/#mediatracksettings-dictionary

BUG=686756, 616098

Review-Url: https://codereview.chromium.org/2664673002
Cr-Commit-Position: refs/heads/master@{#450335}
16 files changed