Benoit Jacob at Mozilla points out in the comments for https://bugs.webkit.org/show_bug.cgi?id=41473 that there are some good unit tests for the CheckedInt class incorporated in that patch ( http://hg.mozilla.org/mozilla-central/file/tip/xpcom/tests/TestCheckedInt.cpp ) which should be added. A reasonable initial location for them would be WebKit/chromium/tests/.
By the way, a couple of minor updates have been made to CheckedInt upstream. Nothing very big but you might still pick them up together with the unit test. Some documentation fixes, and also avoiding relying certain signed integer bit-shift operations that are left undefined by the c++ standard (even though that hasn't been a problem in practice, and the test suite is complete enough to catch any such issue on a given platform).