Leak local static vars in TestPlugin.

In TestPlugin, the static-const-WebString's sometimes crash on exiti when their
destructors are run. This CL forces them to be leaked to avoid this.

BUG=328158

Review URL: https://codereview.chromium.org/115573006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@242396 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 242396