Store custom property backing strings in resolved CSSVariableData objects.
This is a defensive change - it seems unlikely that CSS Variables
can persist beyond the style sheets that create them, but it's also
easy and cheap to ensure that resolved CSS variables pin their
backing strings.
It's worth noting that resolved CSS Variables can *only* reference
either their own string, or strings from other *already resolved*
variables.
BUG=465126
Review URL: https://codereview.chromium.org/1498713002
Cr-Commit-Position: refs/heads/master@{#362913}
2 files changed
