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This CL fixes some colors I noticed were off around the NTP.

* Specify COLOR_DETACHED_BOOKMARK_BAR_SEPARATOR in the GTK theme.  Previously we
  were falling back to a value calculated in [1].
* Gets the download bar separator color from the ThemeProvider.  Previously it
  was hardcoded to #D6D6D6.  This changes it to use the toolbar separator color.
  This color should work well with the download bar because the background color
  of the download bar is already obtained from the toolbar color.

[1] https://cs.chromium.org/chromium/src/chrome/browser/themes/theme_service.cc?rcl=b0784bef916569b6e4f06a6dce95b75a8a2cf10a&l=530

BUG=None

Change-Id: Ib892b8ebb3355299020a4bc570cf37a3aee7a8c2
Reviewed-on: https://chromium-review.googlesource.com/1167979
Commit-Queue: Thomas Anderson <thomasanderson@chromium.org>
Reviewed-by: Peter Kasting <pkasting@chromium.org>
Cr-Commit-Position: refs/heads/master@{#581929}