Add applyConstraints() support for non-device tracks.

This includes screen capture and capture from media/canvas elements.

Bug: 767074
Change-Id: I4f922ac9540fd69dc112cfa80742b511c32a094c
Reviewed-on: https://chromium-review.googlesource.com/698846
Reviewed-by: Henrik Boström <hbos@chromium.org>
Commit-Queue: Guido Urdaneta <guidou@chromium.org>
Cr-Commit-Position: refs/heads/master@{#507518}

• content/browser/webrtc/webrtc_browsertest.cc[diff]
• content/browser/webrtc/webrtc_getusermedia_browsertest.cc[diff]
• content/renderer/media/apply_constraints_processor.cc[diff]
• content/renderer/media/apply_constraints_processor.h[diff]
• content/test/data/media/getusermedia.html[diff]
• content/test/data/media/peerconnection-call.html[diff]