Use EditingTestBase for TextIteratorTest
Use EditingTestBase for TextIteratorTest

This CL changes TextIteratorTest to inherit EditingTestBase to reduce
duplicated code.

BUG=n/a
TEST=n/a; no behavior change

Review URL: https://codereview.chromium.org/1202843007

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@197798 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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