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This color changed recently for the toolkit-views app list, but the
Cocoa app list isn't ready for it yet.

The Cocoa-specific kHighlightedColor was only used in
apps_search_results_controller.mm, so move it there. kSelectedColor is
used there but also in apps_grid_view_item.mm. However, the latter users
a composited draw, so it's fine with the transparent background views
uses.

BUG=469051

Review URL: https://codereview.chromium.org/1023983002

Cr-Commit-Position: refs/heads/master@{#321573}