Make ChromeOS tray background color white instead of off-white.

Patch is designed as a quick and easy fix that should be easy to merge.
A more comprehensive follow up will come later.

BUG=711183

Review-Url: https://codereview.chromium.org/2812223008
Cr-Commit-Position: refs/heads/master@{#465355}
(cherry picked from commit cabc715)

Review-Url: https://codereview.chromium.org/2828053002 .
Cr-Commit-Position: refs/branch-heads/3071@{#56}
Cr-Branched-From: a106f0a-refs/heads/master@{#464641}

Author: Evan Stade

ash/system/tray/tray_constants.cc

@@ -63,7 +63,7 @@ const int kTrayRoundedBorderRadius = 2;
 
 const int kTrayToggleButtonWidth = 68;
 
-const SkColor kBackgroundColor = SkColorSetRGB(0xfe, 0xfe, 0xfe);
+const SkColor kBackgroundColor = SK_ColorWHITE;
 const SkColor kPublicAccountUserCardTextColor = SkColorSetRGB(0x66, 0x66, 0x66);
 const SkColor kPublicAccountUserCardNameColor = SK_ColorBLACK;