Expose accessibility information about find in page results to the client.

All this does is associate the Range of the result with the start and end
accessibility object anf offset.

BUG=426414

Review URL: https://codereview.chromium.org/696533003

git-svn-id: svn://svn.chromium.org/blink/trunk@184890 bbb929c8-8fbe-4397-9dbb-9b2b20218538