Don't layout fixed-pos layer when it's moved

BUG=342538
TEST=fast/repaint/fixed-position-layer-moved.html

Review URL: https://codereview.chromium.org/143073009

git-svn-id: svn://svn.chromium.org/blink/trunk@166896 bbb929c8-8fbe-4397-9dbb-9b2b20218538