Remove many more uses of Page

Mostly replaced them with FrameHost.

Many places in the code are very confused about
when a Frame has a Page or when a Document
has a Frame.  I attempted to replace checks of
Frame or Page with Document::isActive and
ASSERT in the places where the isActive check
shouldn't even be necessary.

I ran into a bug in the animation unittests
assuming that animations were OK on a
frame-less document (which should not be the case).
I filed crbug.com/330095 to track that and
added a comment in the code.

BUG=305811

Review URL: https://codereview.chromium.org/106733004

git-svn-id: svn://svn.chromium.org/blink/trunk@164217 bbb929c8-8fbe-4397-9dbb-9b2b20218538