Exclude cases of full repaint on viewport height change

We treat viewport as the background positioning area if background-
attachment is 'fixed' (otherwise the viewport is only the background
paiting area, not the background positioning area).

Added the condition to avoid full repaint if background is not fixed
or is composited.

BUG=258219
TEST=fast/repaint/window-resize-background-image-generated.html
TEST=fast/repaint/window-resize-background-image-non-fixed.html
TEST=fast/repaint/window-resize-background-image-fixed-centered-composited.html

Review URL: https://codereview.chromium.org/293063016

git-svn-id: svn://svn.chromium.org/blink/trunk@175398 bbb929c8-8fbe-4397-9dbb-9b2b20218538