Split up the if checks in ComputedStyle to make them easy to generate

This patch splits up the iff checks in the diffing function
ComputedStyle::UpdatePropertySpecificDifferences with the intention of
making it easy to generate with the macro fieldwise_diff created here:
https://codereview.chromium.org/2858863002

As can be seen in the try bot runs this has no perf impact

BUG=710938

Review-Url: https://codereview.chromium.org/2855873003
Cr-Commit-Position: refs/heads/master@{#469268}
1 file changed