Unified: Update green color in a11y detailed view.

According to UX spec, check icon shown on the right side of a11y
detailed view is 28994F. This change is important in terms of color
contrast.

TEST=manual
BUG=899162

Change-Id: Idd32cbf94d974355a6229ef7bb88e9ae714ab97c
Reviewed-on: https://chromium-review.googlesource.com/c/1304284Reviewed-by: Yoshiki Iguchi <yoshiki@chromium.org>
Commit-Queue: Tetsui Ohkubo <tetsui@chromium.org>
Cr-Commit-Position: refs/heads/master@{#603764}

ash/system/tray/tray_popup_utils.cc

@@ -367,7 +367,7 @@ bool TrayPopupUtils::CanOpenWebUISettings() {
 void TrayPopupUtils::InitializeAsCheckableRow(HoverHighlightView* container,
                                               bool checked) {
   gfx::ImageSkia check_mark =
-      CreateVectorIcon(kCheckCircleIcon, gfx::kGoogleGreen700);
+      CreateVectorIcon(kCheckCircleIcon, gfx::kGoogleGreenDark600);
   container->AddRightIcon(check_mark, check_mark.width());
   UpdateCheckMarkVisibility(container, checked);
 }