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).