Fix an issue that subtitle is not shown for native fullscreen video that is inside an iframe

Subtitle support for native fullscreen video is enabled in https://codereview.chromium.org/22454003/.
However, there are a couple bugs:
1. FullscreenElementStack::currentFullScreenElementFrom() returns NULL if the 
fullscreen element is in a child frame, we need to use 
FullscreenElementStack::fullscreenElementFrom().

2. When the video element is appended to the top level frame, we should exclude 
it when rebuilding the compositing layer tree.

A new layout test is added: fullscreen/full-screen-iframe-allowed-video.html

This CL also fixes a bug that some of the fullscreen pixel tests are no longer 
running due to  https://codereview.chromium.org/24438004

BUG=349222,347843

Review URL: https://codereview.chromium.org/187193002

git-svn-id: svn://svn.chromium.org/blink/trunk@168578 bbb929c8-8fbe-4397-9dbb-9b2b20218538