dark_mode: Add kButtonLabelColorBlue.

This specific color will be used by system toast.

Bug: 1107058
Change-Id: I420f78be163cfb012abaf555915a89fbc591f432
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2414560Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
Commit-Queue: Min Chen <minch@chromium.org>
Cr-Commit-Position: refs/heads/master@{#808662}

ash/style/ash_color_provider.cc

@@ -242,6 +242,8 @@ SkColor AshColorProvider::GetContentLayerColor(ContentLayerType type) const {
     case ContentLayerType::kButtonLabelColor:
     case ContentLayerType::kButtonIconColor:
       return is_dark_mode ? gfx::kGoogleGrey200 : gfx::kGoogleGrey700;
+    case ContentLayerType::kButtonLabelColorBlue:
+      return is_dark_mode ? gfx::kGoogleBlue300 : gfx::kGoogleBlue600;
     case ContentLayerType::kButtonLabelColorPrimary:
     case ContentLayerType::kButtonIconColorPrimary:
       return is_dark_mode ? gfx::kGoogleGrey200 : gfx::kGoogleGrey900;

ash/style/ash_color_provider.h

@@ -113,6 +113,9 @@ class ASH_EXPORT AshColorProvider : public SessionObserver {
     kButtonLabelColor,
     kButtonLabelColorPrimary,
 
+    // Color for blue button labels, e.g, 'Retry' button of the system toast.
+    kButtonLabelColorBlue,
+
     kButtonIconColor,
     kButtonIconColorPrimary,