ScreenCapture for Android phase1, part II

The capture part has been landed in cl https://codereview.chromium.org/2125973002/.
This cl is to implement the control part in chrome/ and content/. The document
is here, https://goo.gl/QNH29g.
This cl proposes a pemission design on Android, including permission info bar
and head up notification, which will show up each time at starting screen capture
and user can check/stop the capture without leaving current page.
And all of these is behind a flag as an experimental feature.

BUG=487935

Review-Url: https://codereview.chromium.org/2123863004
Cr-Commit-Position: refs/heads/master@{#420679}
22 files changed