Use const CSSParserContext reference at some places where it's never null

Change-Id: I1deab83b451783a6db930a1c6b98055bba57d8f1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2055684
Reviewed-by: Anders Hartvoll Ruud <andruud@chromium.org>
Commit-Queue: Xiaocheng Hu <xiaochengh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#741471}
9 files changed