Don't assume the root renderer covers the background when it has filters

In r166582 I added an optimization for the RenderView to avoid double-painting
the entire viewport background when the root renderer fully obscures the view.
That patch missed a case where the root renderer has an opacity filter, which
left the root background partially transparent. This patch disables the
RenderView optimization when the root has a filter applied.

BUG=349587

Review URL: https://codereview.chromium.org/196473002

git-svn-id: svn://svn.chromium.org/blink/trunk@168980 bbb929c8-8fbe-4397-9dbb-9b2b20218538