Don't require main-thread scrolling for Gmail

In Gmail, the FrameView doesn't scroll, which means we don't create a
composited layer for fixed position elements. However, when checking for
main-thread scrolling reasons, we'd notice these non-composited fixed-position
elements and think that meant we needed main-thread scrolling.

This CL skips the check for non-composited fixed-position elements if the
FrameView isn't scrollable. If the FrameView isn't scrollable, there's no
reason to force main thread scrolling in this case.

R=vollick@chromium.org
BUG=404332

Review URL: https://codereview.chromium.org/480573007

git-svn-id: svn://svn.chromium.org/blink/trunk@180688 bbb929c8-8fbe-4397-9dbb-9b2b20218538