[Blink, RemotePlayback] Added the disableRemotePlayback check to prompt().

BUG=647441
TEST=avayvod.github.io/remote-playback/test.html and a layout test

Review-Url: https://codereview.chromium.org/2348703002
Cr-Commit-Position: refs/heads/master@{#421436}
2 files changed