Fix MergeAndOverrideOnConflict for custom properties.

Utilizing CSSPropertyName, to make the code much nicer.

R=futhark@chromium.org

Change-Id: I10733cabe27cc271eaaf3ecf62fa7713cf74eac3
Reviewed-on: https://chromium-review.googlesource.com/c/1323650
Commit-Queue: Anders Ruud <andruud@chromium.org>
Reviewed-by: Rune Lillesveen <futhark@chromium.org>
Cr-Commit-Position: refs/heads/master@{#606847}
6 files changed