Route selection bounds updates through WebLayerTreeView

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

Instead, plumb the selection bounds region through the WebLayerTreeView,
providing appropriate composited layers for each endpoint.  This allows the
compositor to transform the bounds as necessary to provide consistent
positioning of visible selection handles.

BUG=135959

Review URL: https://codereview.chromium.org/454643002

git-svn-id: svn://svn.chromium.org/blink/trunk@182460 bbb929c8-8fbe-4397-9dbb-9b2b20218538