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We do it in two stages, first a DOM element requests fullscreen inside <webview>,
this fires a permissionrequest to the embedder of <webview> to decide
whether to allow/deny the request. The element goes fullscreen within
the <webview>'s bounds at this point.
The embedder can:
a) allow the request: in this case we perform the second stage, where the
<webview> element in the embedder enters fullscreen (provided that
the embedder has permission to enter fullscreen).
b) deny the request: in this case, <webview> exits fullscreen mode.

Note that b) will cause a flicker and blink interface is required to avoid that.
That is tracked on http://crbug.com/466854.

BUG=141198
Test=
Load a chrome app with a <webview> in it, make the <webview> point to
some site that has html5 fullscreen request, e.g. youtube.com.

Add an event listener in the chrome app so that it allows <webview> fullscreen, e.g.

<webview>.addEventListener('permissionrequest', function(e) {
  if (e.permission === 'fullscreen') e.request.allow();
});

Now perform necessary action to request fullscreen from the <webview>, for the
youtube case, this would be fullscreening the video player.

Check that the element enters fullscreen mode.

Review URL: https://codereview.chromium.org/984963004

Cr-Commit-Position: refs/heads/master@{#321282}