Support clearing of composited selection bounds

When composited selection updates are enabled, the compositor should push
selection updates as necessary when updating the tree. As an initial step to
this end, expose a pipeline for pushing updates when the selection has been
cleared. Also add this as a runtime feature, to be made public when proper
selection bound computation is added.

BUG=135959

Review URL: https://codereview.chromium.org/377603002

git-svn-id: svn://svn.chromium.org/blink/trunk@177855 bbb929c8-8fbe-4397-9dbb-9b2b20218538