Update spelling|grammar error markers rendering on Mac

Mac platform used to have a gradient on spelling|grammar error markers,
but that's no longer the case anymore.
This patch removes such gradient and also updates the colors
so they match the ones used by WebKit on Mac.

Change-Id: Ia6cd75a0c70f6ea4884493f6b8e69de48bf92b4e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3139819


Reviewed-by: Philip Rogers <pdr@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Commit-Queue: Manuel Rego <rego@igalia.com>
Cr-Commit-Position: refs/heads/main@{#918855}