[DevTools] Extract ScreencastFrameMetadata in protocol.json.

Step 2 of 3.

BUG=318188

Review URL: https://codereview.chromium.org/70293002

git-svn-id: svn://svn.chromium.org/blink/trunk@162626 bbb929c8-8fbe-4397-9dbb-9b2b20218538