Fix some memory leaks in Blink unit tests.

Found by LeakSanitizer. I manually audited similar usages elsewhere in
Blink to make sure we weren't leaking it ouside of tests too.

BUG=none
R=abarth@chromium.org

Review URL: https://codereview.chromium.org/177093014

git-svn-id: svn://svn.chromium.org/blink/trunk@167759 bbb929c8-8fbe-4397-9dbb-9b2b20218538