We should consider merging this change https://codereview.chromium.org/15317009 and the test case which is not currently in WebKit.
There is no need to merge this fix. The ASSERT() was triggered because the code was using isEqualToCSSIdentifier() which had some restrictions not applicable to the CSS variables case. Actually the ASSERT() mentioned in the Chromium bug can no longer happen since r150776, as we perform a case sensitive check in a different inline method which does not have those assertions.