[Autofill Cleanup] Use NativeTheme Colors

This CL addreses one of the post-launch cleanup items for the Autofill
Views rewrite. Specifically, it replaces hardcoded colors with colors
taken from the NativeTheme.

The new colors are all defined in common_theme and, with one exception,
are equal to the hardcoded ones they are replacing. The exception is
kColorId_MenuSeparatorColor, which this CL updates from a neutral grey
to the corresponding GoogleGrey value. This is consistent with the spec
for Autofill, and I believewe should be using it Chrome-wide as well.

This will also allow work to proceed on macOS Dark Mode implementation.

Bug: 831603
Change-Id: I6755285cff9096daef3e468a407214dc5dd770bc
Reviewed-on: https://chromium-review.googlesource.com/c/1367936
Commit-Queue: Tommy Martino <tmartino@chromium.org>
Reviewed-by: Vasilii Sukhanov <vasilii@chromium.org>
Reviewed-by: Evan Stade <estade@chromium.org>
Reviewed-by: Leonard Grey <lgrey@chromium.org>
Cr-Commit-Position: refs/heads/master@{#615326}