MediaStream API: Refactoring MediaConstraints to use the new style
Instead of the old core/module/mediastream, platform/mediastream and
public/platform/ pattern the platform/mediastream are removed and
the rest of the files refactored accordingly.

Review URL: https://codereview.chromium.org/135853020

git-svn-id: svn://svn.chromium.org/blink/trunk@165241 bbb929c8-8fbe-4397-9dbb-9b2b20218538