Remove desktop role usage

The desktop role gets used as the root of the entire automation tree and should not be used elsewhere.

This change adds a comment in the enums file where views authors hopefully look for the list of roles.

Note that in ChromeVox, using the desktop role amounts to a no-op, since ChromeVox only checks desktop role when it asks for a AutomationNode.prototype.root.

TBR=tsepez@chromium.org

Bug: 787101
Change-Id: I9f1b7a7a71f69f43ebc265cf0b61d0b0bdb4249f
Reviewed-on: https://chromium-review.googlesource.com/896557
Commit-Queue: David Tseng <dtseng@chromium.org>
Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#534514}

ui/accessibility/ax_enums.mojom

@@ -78,6 +78,12 @@ enum Event {
   kLast = kValueChanged,
 };
 
+// Explanation:
+// The majority of these roles come from the ARIA specification. Reference
+// the latest draft for proper usage.
+//
+// Roles not included by the ARIA specification should be avoided, especially
+// internal roles used by the accessibility infrastructure.
 enum Role {
   kNone,
   kAbbr,
@@ -110,7 +116,7 @@ enum Role {
   kDescriptionListDetail,
   kDescriptionList,
   kDescriptionListTerm,
-  kDesktop,
+  kDesktop,  // internal
   kDetails,
   kDialog,
   kDirectory,

ui/app_list/views/app_list_view.cc

@@ -991,7 +991,7 @@ void AppListView::Layout() {
 
 void AppListView::GetAccessibleNodeData(ui::AXNodeData* node_data) {
   node_data->SetName(state_announcement_);
-  node_data->role = ax::mojom::Role::kDesktop;
+  node_data->role = ax::mojom::Role::kAlert;
 }
 
 void AppListView::OnKeyEvent(ui::KeyEvent* event) {