Unified: Change tray icon color to be fitted to the spec.

The color of active icons should be Grey 200, which is darker than the
labels by 1 level.
This will affect multiple places including:
- tray collapse button
- feature pod icons
- top shortcut buttons
- slider button
- menu back arrow icon
- ink drop of sign out button
- tray popup

Bug: 853695
Change-Id: Ifbbfe221db040f2f02d9577a636b7c7dd4f796f0
Reviewed-on: https://chromium-review.googlesource.com/1180787Reviewed-by: Tetsui Ohkubo <tetsui@chromium.org>
Reviewed-by: Yoshiki Iguchi <yoshiki@chromium.org>
Commit-Queue: Tatsuhisa Yamaguchi <yamaguchi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#585363}

ash/system/tray/tray_constants.h

@@ -140,7 +140,7 @@ constexpr SkColor kUnifiedMenuBackgroundColorWithBlur =
     SkColorSetA(kUnifiedMenuBackgroundColor, 0x99);
 constexpr float kUnifiedMenuBackgroundBlur = 30.f;
 constexpr SkColor kUnifiedMenuTextColor = SkColorSetRGB(0xf1, 0xf3, 0xf4);
-constexpr SkColor kUnifiedMenuIconColor = SkColorSetRGB(0xf1, 0xf3, 0xf4);
+constexpr SkColor kUnifiedMenuIconColor = SkColorSetRGB(0xe8, 0xea, 0xed);
 constexpr SkColor kUnifiedMenuSecondaryTextColor =
     SkColorSetA(kUnifiedMenuIconColor, 0xa3);
 constexpr SkColor kUnifiedMenuIconColorDisabled =