GitLab

The overflow shelf was incorrectly hard coded to use a black background. This
change updates the overflow shelf color based on the wallpaper color.

BUG=719724
TEST=ash_unittest --gtest_filter=ShelfViewTest.OverflowShelfColorIsDerivedFromWallpaper

Review-Url: https://codereview.chromium.org/2867413002
Cr-Commit-Position: refs/heads/master@{#470703}