RenderLayerCompositor::updateLayerIfViewportConstrained can ASSERT incorrectly

If a RenderLayer goes from being fixed position to be absolutely positioned,
its viewportConstrainedNotCompositedReason is supposed to change to
NoNotCompositedReason. Previously we would ASSERT in this situation.

R=vollick@chromium.org
BUG=365668

Review URL: https://codereview.chromium.org/251593007

git-svn-id: svn://svn.chromium.org/blink/trunk@172701 bbb929c8-8fbe-4397-9dbb-9b2b20218538