[Mac] Tabpose: Add a NULL check before trying to query for a devtools TabContents.
[Mac] Tabpose: Add a NULL check before trying to query for a devtools TabContents.

BUG=77644
TEST=Open Tabpose. Don't crash.

Review URL: http://codereview.chromium.org/6724037

git-svn-id: dd90618784b6a4b323ea0c23a071cb1c9e6f2ac7@79563 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser