FrameView::updateWidgets() can be made simpler

The widget updating code was all done in terms of RenderObject* and then
checked for isEmbeddedObject() but it turns out it only ever interacts
with RenderEmbeddedObjects so we can tighten the types and simplify the
code a whole bunch like removing all the manual ref count management
and merging the function into the loop now that it's much simpler.

Review URL: https://codereview.chromium.org/109943004

git-svn-id: svn://svn.chromium.org/blink/trunk@163490 bbb929c8-8fbe-4397-9dbb-9b2b20218538