Renamed InspectorDOMAgent::loadEventFired to InspectorDOMAgent::invalidateFrameOwnerElement and added
a call from InspectorDOMAgent::didCommitLoad.

This is to make sure that it will invalidate the content of the iframe as soon as the frame navigates
to a different page. This way the new node can be retrieved as soon as the page has some content, and
not just when the page is fully loaded.

BUG=307280

Review URL: https://codereview.chromium.org/27263002

git-svn-id: svn://svn.chromium.org/blink/trunk@159792 bbb929c8-8fbe-4397-9dbb-9b2b20218538