Move code in ComputedStyle::UpdatePropertySpecificDifferences to generate diffing helpers

This patch moves code around in
ComputedStyle::UpdatePropertySpecificDifferences to allow us to easily
generate one diffing function helper per flag to be set on
StyleDifference.

BUG=710938

Review-Url: https://codereview.chromium.org/2900973004
Cr-Commit-Position: refs/heads/master@{#474571}
1 file changed