Allow active sheets to have nullptr for RuleSet.

CSSStyleSheets which have a non-matching media attribute may have a
nullptr for RuleSet since it's not needed. Handle that in active style-
sheet diffing. That means adding a sheet with non-matching media should
not cause any style recalculations.

R=meade@chromium.org
BUG=567021

Review-Url: https://codereview.chromium.org/2401573002
Cr-Commit-Position: refs/heads/master@{#423804}
2 files changed