Fix memory leak in Source/testing/plugin/main.cpp.

This CL adds a missing free() for 'obj->evaluateScriptOnMouseDownOrKeyDown'.

Review URL: https://codereview.chromium.org/46043009

git-svn-id: svn://svn.chromium.org/blink/trunk@160830 bbb929c8-8fbe-4397-9dbb-9b2b20218538