Fix Ash app title bar color being blue instead of grey

The color of Ash app window title bar was changed to
the same color of the settings app title bar accidentally in
https://chromium-review.googlesource.com/c/chromium/src/+/1125512.
This changed DevTools, the Terminal and Chrome App windows to be blue.

This CL fixes the unintentional change in behavior by updating
UpdateFrameColors() to use the same guard as CreateFrameHeader() when
setting the frame color to kMdWebUiFrameColor.

Before: https://bugs.chromium.org/p/chromium/issues/attachment?aid=359594&signed_aid=Yr-u3b9GUhWb0iEc6igy5Q==&inline=1
After: https://bugs.chromium.org/p/chromium/issues/attachment?aid=359595&signed_aid=rfGnleo9Sj8jmcVXfDipeg==&inline=1

Bug: 879851
Change-Id: I8980b7f52f529390938a1a5ff05742b336e665d2
Reviewed-on: https://chromium-review.googlesource.com/1223951
Commit-Queue: Alan Cutter <alancutter@chromium.org>
Reviewed-by: Evan Stade <estade@chromium.org>
Reviewed-by: Bret Sepulveda <bsep@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#593802}(cherry picked from commit 5becfacf4db983efdd63625cfaad8693e60e13c3)
Reviewed-on: https://chromium-review.googlesource.com/1247721
Reviewed-by: Alan Cutter <alancutter@chromium.org>
Cr-Commit-Position: refs/branch-heads/3538@{#694}
Cr-Branched-From: 79f7c91a2b2a2932cd447fa6f865cb6662fa8fa6-refs/heads/master@{#587811}