Remove superfluous diff code in ComputedStyle::UpdatePropertySpecificDifferences

This code was generated in https://codereview.chromium.org/2897193005
and the generated function is being called too. I seem to have forgotten
to remove the handwritten code. The diff of the generated code is here:
https://gist.github.com/7db928ec119076f5526f3f6d2117d407/revisions

BUG=710938

Review-Url: https://codereview.chromium.org/2920943002
Cr-Commit-Position: refs/heads/master@{#476603}
1 file changed