Fix fullscreen elements in pinch viewport mode.

In pinch viewport mode, the size of the FrameView is always the content
width of the document (not the true viewport size), so this patch plumbs
the viewport size to fullscreen elements separately.  Secondly, in this
mode there is an "inner viewport" that scrolls separately and whose
scrollability additionally needs to be disabled.  Do so by using
setUserScrollable() (which is a mechanism that only works in this mode).

BUG=411072

Review URL: https://codereview.chromium.org/636363003

git-svn-id: svn://svn.chromium.org/blink/trunk@183623 bbb929c8-8fbe-4397-9dbb-9b2b20218538