[TextBlob] Add support for offsets (i.e. complex text) to text blobs.

BUG=377845
TEST=No regression in layout tests

Review URL: https://codereview.chromium.org/667573008

git-svn-id: svn://svn.chromium.org/blink/trunk@184121 bbb929c8-8fbe-4397-9dbb-9b2b20218538