[TextBlob] Add support for offsets (i.e. complex text) to text blobs.
[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: bf5cd6ccde378db821296732a091cfbcf5285fbd@184121 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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