Navigation transitions: Initiate navigation transition and send serialized markup to Chrome

If any transition-elements metatags are present, we signal Chrome that this is a transition navigation and serialize the HTML elements designated by the selectors in the transitions-elements metatag.

The isTransitionNavigation flag needs to be passed through both WebFrameClient::decidePolicyForNavigation and WebFrameClient::dispatchDidStartProvisionalLoad to cover both same-origin and different-origin navigation paths.

Requires https://codereview.chromium.org/281383006/

R=esprehn,eseidel
BUG=281383006

Review URL: https://codereview.chromium.org/319573008

git-svn-id: svn://svn.chromium.org/blink/trunk@178413 bbb929c8-8fbe-4397-9dbb-9b2b20218538