CompositedLayerMapping::setNeedsGraphicsLayerUpdate shouldn't dirty up to the root.

There's no longer any reason to mark dirty bits up the CompositedLayerMapping
tree. Also, given that we walk the entire tree in GraphicsLayerUpdater, we
don't even need breadcrumbs up the tree to find the mappings that need updates.

R=chrishtr@chromium.org

Review URL: https://codereview.chromium.org/337253003

git-svn-id: svn://svn.chromium.org/blink/trunk@176556 bbb929c8-8fbe-4397-9dbb-9b2b20218538