Add unit test for base::scoped_refptr boolean operations.

Added unit test for base::scoped_refptr boolean operations.
Also added a comment to 'operator T*() const' in order to make it more apparent to the User that boolean operations with scoped_refptr are possible without invoking 'get()' method.

Review URL: https://codereview.chromium.org/26868003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@229802 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 229802