[Telemetry] Replace assertGreater with assertTrue.
[Telemetry] Replace assertGreater with assertTrue.

unittest.assertGreater doesn't exist in Python 2.6. Reported by achuith.

BUG=None.
TEST=None.

Review URL: https://codereview.chromium.org/11929026

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@177681 0039d316-1c4b-4281-b951-d872f2087c98

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