Rename isRenderingMaskImage to isRenderingClipPathAsMaskImage

Also rename the PaintBehavior flag PaintBehaviorRenderingSVGMask
to PaintBehaviorRenderingClipPathAsMask.

BUG=420022

Review URL: https://codereview.chromium.org/623243003

git-svn-id: svn://svn.chromium.org/blink/trunk@183272 bbb929c8-8fbe-4397-9dbb-9b2b20218538