Revert "Refactoring: Clarifying DOMWindow ownership by removing Frame::setDOMWindow()"

This reverts r154885 since it introduces a difference in how DOMWindow
is handled if FrameLoader::clear() is re-entered. Before r154885, we
would always assign the frame a new DOMWindow if it wasn't the initial
navigation away from about:blank. After r154885 though, re-entering
FrameLoader::clear() from DocumentLoader::createWriterFor() would end up
reusing the same DOMWindow, since m_needsClear would be false in that
case.

Review URL: https://chromiumcodereview.appspot.com/21296004

git-svn-id: svn://svn.chromium.org/blink/trunk@155226 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Move slow tests into a SlowTests file.

Now that expectations files union conflicting lines, we can move
Slow lines into their own file. This both reduces noise in the
main TestExpectations file and make Slow tests easier to manage, e.g.
if the test starts failing you don't need to go through hoops to
avoid conflicting with the existing Slow line.

TBR=dpranke@chromium.org

Review URL: https://chromiumcodereview.appspot.com/21179004

git-svn-id: svn://svn.chromium.org/blink/trunk@155227 bbb929c8-8fbe-4397-9dbb-9b2b20218538