[ChromeDriver] Add Sam to OWNERS file.

Sam has been working on chromedriver since Oct. 2013 and he has contributed over 50 CLs and reviewed around 20 CLs.
So I recommend that we add Sam as an owner of chromedriver.

R=craigdh@chromium.org

Review URL: https://codereview.chromium.org/469523002

Cr-Commit-Position: refs/heads/master@{#289120}
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@289120 0039d316-1c4b-4281-b951-d872f2087c98

chrome/test/chromedriver/OWNERS

 craigdh@chromium.org
 frankf@chromium.org
+samuong@chromium.org
 stgao@chromium.org