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Currently all tests for TranslateHelper are implemented as browser tests.
But some small tests can run as unit tests. This change introduce new unittest
file for TranslateHelper, then replace an existing test into it.

BUG=175673
TEST=unit_tests --gtest_filter='TranslateHelperTest.*'

Review URL: https://codereview.chromium.org/12209114

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@182412 0039d316-1c4b-4281-b951-d872f2087c98