Removes tab_unittest.cc test parameterization for rtl
Removes tab_unittest.cc test parameterization for rtl

The rtl specific bits of Tab are no longer covered by these tests, so
that there is no point in running two versions.

BUG=none
TEST=test only change

Review URL: https://codereview.chromium.org/1603863006

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#370609}

axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk