Blink side changes to support asynchronous dictionary lookup.

Add support for finding the word under a point and compute its lower left
baseline point.

BUG=121917

Review URL: https://codereview.chromium.org/184963011

git-svn-id: svn://svn.chromium.org/blink/trunk@169094 bbb929c8-8fbe-4397-9dbb-9b2b20218538