Image Capture: cache constraints and append them to MediaStreamTrack.getConstraints()

This CL teaches Image Capture to cache the constraints passed via
applyConstraints() and wires MediaStreamTrack::getConstraints() to
append those upon request.

Note that constraints are accumulative, i.e. two calls to
applyConstraints() with different constraints will essentially
be understood by the platform as a single applyConstraints()
call with the constraint dictionaries merged.

(Also moved MediaStreamTrack::getConstraints() to follow the
declaration order in MediaStreamTrack.h).

Added LayoutTests.


Cr-Commit-Position: refs/heads/master@{#458183}
4 files changed