Don't calculate a top controls adjustment if pinch viewport size is uninitialized.

This was causing a NaN assertion further on in FrameView::maximumScrollPosition.

BUG=425757

Review URL: https://codereview.chromium.org/663993003

git-svn-id: svn://svn.chromium.org/blink/trunk@184298 bbb929c8-8fbe-4397-9dbb-9b2b20218538