GMC: Use correct popup type for a11y

This CL updates the a11y popup type for the media toolbar button to fix
an issue where JAWS would tell the user to use arrow keys instead of
the tab key.

Bug: 1022971
Change-Id: I6523bdd9380ec61cfbb05a86923c654877e818e2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1906636
Commit-Queue: Aaron Leventhal <aleventhal@chromium.org>
Reviewed-by: Aaron Leventhal <aleventhal@chromium.org>
Cr-Commit-Position: refs/heads/master@{#714669}

chrome/browser/ui/views/global_media_controls/media_toolbar_button_view.cc

@@ -42,7 +42,7 @@ MediaToolbarButtonView::MediaToolbarButtonView(const Browser* browser)
   EnableCanvasFlippingForRTLUI(false);
   SetTooltipText(
       l10n_util::GetStringUTF16(IDS_GLOBAL_MEDIA_CONTROLS_ICON_TOOLTIP_TEXT));
-  GetViewAccessibility().OverrideHasPopup(ax::mojom::HasPopup::kMenu);
+  GetViewAccessibility().OverrideHasPopup(ax::mojom::HasPopup::kDialog);
 
   ToolbarButton::Init();