Create a message loop if needed in CookieStore unittests.
Create a message loop if needed in CookieStore unittests.

The cookie_store_unittest.h file can be used outside of the net test suite. In
this case, it is possible that no message loop is running. This CL lazily
creates a MessageLoop if needed.

BUG=NONE
TEST=NONE

Review URL: https://chromiumcodereview.appspot.com/9361023

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@121212 0039d316-1c4b-4281-b951-d872f2087c98

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