Don't invalidate scrolling container layer when scrolling contents need invalidation

Add CompositedLayerMapping::setScrollingContentsNeedsDisplay() and
CompositedLayerMapping::setScrollingContentsNeedsDisplayRect() to
distinguish invalidation of scrolling contents from invalidation of
the container.

BUG=416535
TBR=esprehn@chromium.org (Source/web/LinkHighlight.cpp related to composited scrolling layer)

Review URL: https://codereview.chromium.org/596643002

git-svn-id: svn://svn.chromium.org/blink/trunk@184667 bbb929c8-8fbe-4397-9dbb-9b2b20218538