[DevTools] Remove temporary duplicated properties in
renderer_overrides_handler to finish ScreencastFrameMetadata extraction.

Step 3 of 3.

BUG=318188

Review URL: https://codereview.chromium.org/64833005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@237801 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 237801