Expose public interface for composited selection bounds

Currently, the selection bounds are pulled from the WebView at the start of each
frame. Such an approach works properly only if the queried bounds are always
in-sync with the visible content, which will not always hold with accelerated
compositing.

This patch exposes a public interface for enabling and routing composited
selection bounds. Metadata for the composited bounds provides the compositor
with all necessary information to reposition and determine visibility of the
selection endpoints. The data is exposed via the WebLayerTreeView, and will
ultimately be provided by the RenderLayerCompositor.

Note that implementation of the composited bounds computation will be performed
in a separate follow-up patch (see https://codereview.chromium.org/302993003/).

BUG=135959

Review URL: https://codereview.chromium.org/352173002

git-svn-id: svn://svn.chromium.org/blink/trunk@176958 bbb929c8-8fbe-4397-9dbb-9b2b20218538