Hook up style invalidation for CSS Paint API.

This introduces the plumbing required invalidating a custom paint
function.

This breaks the style invalidation into two distinct sets.
 - If the property is a valid native property the registerPaint function
   will convert it to a CSSPropertyID.
 - If it is a CSS variable (custom property) it will convert to an
   AtomicString.

The invalidation check occurs in ComputedStyle::diffNeedsPaintInvalidationObject

This relies on the incomplete behaviour of CSSPropertyEquality which
needs to be auto-generated (more).

BUG=578252

Review-Url: https://codereview.chromium.org/1896893004
Cr-Commit-Position: refs/heads/master@{#394223}
34 files changed