Fixed position layers go in an out of compositing mode on each frame.

If the fixed element is offscreen, but pushed on-screen by a transform
it will pop in and out of compositing mode for each frame because
it doesn't apply transforms if it's composited. The fix is to
always apply transforms if we're in the ApplyBoundsChickenEggHacks
codepath since the only place we pass that in is in the fixed
position check in CompositingReasonFinder::requiresCompositingForPositionFixed,
where we always want to pretend that the element is composited.

I'm not sure why the ancestorLayer == this check was there in the
first place. It was added in https://codereview.chromium.org/226043007,
but there's no explanation in the code review for why it was there.
I don't see how that case is even possible since it would
only ever be true if we had a fixed position root render layer and
were checking it's bounds, which can't happen.

BUG=354100

Review URL: https://codereview.chromium.org/299343002

git-svn-id: svn://svn.chromium.org/blink/trunk@174750 bbb929c8-8fbe-4397-9dbb-9b2b20218538