Remove unnecessary Context::Scope from V8CSSStyleRuleCustom

This is a left-over of https://codereview.chromium.org/1683493003/.
This CL removes it.

BUG=455160, 610340

Review-Url: https://codereview.chromium.org/2024793002
Cr-Commit-Position: refs/heads/master@{#396808}
1 file changed