Add missing --google-gray-900 color.

c/s/c/c/r/interstitial_common.css uses --google-gray-900, but it's not
defined anywhere. Adding it to c/s/c/c/r/interstitial_core.css

Bug: NONE
Change-Id: I2a0a699337285c7c3fe2338165205390a81ed4a7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1548634
Reviewed-by: Edward Jung (EMEA) <edwardjung@chromium.org>
Reviewed-by: Carlos IL <carlosil@chromium.org>
Commit-Queue: Carlos IL <carlosil@chromium.org>
Auto-Submit: Hector Carmona <hcarmona@chromium.org>
Cr-Commit-Position: refs/heads/master@{#646874}
1 file changed