Provide CSSPropertyID HashTraits in the same file

This avoids any subtle problems from neglecting to #include the
file that defines the HashTraits.

Fixed: 1246970
Change-Id: If1c6be4b7400c784b50c74598d080b3a47c96c83
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3141882


Reviewed-by: Omer Katz <omerkatz@chromium.org>
Commit-Queue: Anders Hartvoll Ruud <andruud@chromium.org>
Cr-Commit-Position: refs/heads/main@{#918740}