[Harmony Cast Dialog] Give the "no devices found" info button a label

Give the info button a label read by screen readers when it is in focus:
"No Cast destinations found. Need help?"

Bug: 869600
Change-Id: I90af935d528f7aa128469b98ecb691f6d13228a0
Reviewed-on: https://chromium-review.googlesource.com/1214044Reviewed-by: Derek Cheng <imcheng@chromium.org>
Commit-Queue: Takumi Fujimoto <takumif@chromium.org>
Cr-Commit-Position: refs/heads/master@{#590017}

chrome/browser/ui/views/media_router/cast_dialog_no_sinks_view.cc

@@ -87,6 +87,8 @@ views::View* CastDialogNoSinksView::CreateHelpIconView() {
   help_icon->SetFocusForPlatform();
   help_icon->SetBorder(
       views::CreateEmptyBorder(gfx::Insets(kPrimaryIconBorderWidth)));
+  help_icon->SetAccessibleName(
+      l10n_util::GetStringUTF16(IDS_MEDIA_ROUTER_DESTINATION_MISSING));
   HoverButton* view =
       new HoverButton(/* button_listener */ nullptr, std::move(help_icon),
                       title, base::string16());