Unify "contributes to" and "requires mask" for clip-path child iteration

Since contributesToClip(...) and requiresMask(...) have a lot of overlap,
refactor them into a new (set of) function(s) that return an enumeration
based on the requirements for the element in question.

Review-Url: https://codereview.chromium.org/2563613002
Cr-Commit-Position: refs/heads/master@{#437248}
1 file changed