Re-enable the activation behavior of media elements (click to play/pause)

This revives most of the code and tests from an earlier revert:
https://codereview.chromium.org/249483002

The "Don't fire events when interacting with <video controls>" bug has
been fixed, so the native controls and scripted controls will no
longer compete for control:
https://codereview.chromium.org/406213002

The definition of "restrained media controller" has been simplified in
spec and implementation since the last time:
https://codereview.chromium.org/262463003

BUG=354746

Review URL: https://codereview.chromium.org/830183005

git-svn-id: svn://svn.chromium.org/blink/trunk@188141 bbb929c8-8fbe-4397-9dbb-9b2b20218538
8 files changed
tree: 4e2178be5450d5fc952545178b8c1126fd596485
  1. third_party/