[RenderText] Added chrome os exclusion from doesntClip test

[RenderText] Added chrome os exclusion from doesntClip test.

The GetStringSize has had some unfound errors that are now being found by an improvement to the unit test.  There is another bug (465583) that is intended to look into these standing issues.  For the short term, this CL will remove Chrome OS from the failing unit test (which is no worst than the situation prior to the unit test being improved).

BUG=468903
Committed: https://crrev.com/294db37df3285042a677d49c3b4040250d0c5c97
Cr-Commit-Position: refs/heads/master@{#321590}

[dschuyler, 2015-03-19 20:54:36]

dschuyler@chromium.org changed reviewers:
+ sadrul@chromium.org

[sadrul, 2015-03-20 04:02:37]

sadrul@chromium.org changed reviewers:
+ msw@chromium.org
- sadrul@chromium.org

[sadrul, 2015-03-20 04:02:37]

msw@ is render-text owner

[msw, 2015-03-20 16:40:24]

shrug, lgtm

[dschuyler, 2015-03-20 17:10:21]

The CQ bit was checked by dschuyler@chromium.org

[commit-bot: I haz the power, 2015-03-20 17:11:12]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1003273004/1

[commit-bot: I haz the power, 2015-03-20 17:35:01]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-03-20 17:36:17]

Patchset 1 (id:??) landed as
https://crrev.com/294db37df3285042a677d49c3b4040250d0c5c97
Cr-Commit-Position: refs/heads/master@{#321590}