Fire AXNewDocumentLoadComplete rather than AXLoadComplete.

This fixes a bug where when a page loads, Voiceover focus shifts to
the main content and starts reading it, regardless of whether the
"Automatically speak the webpage" option is checked in VoiceOver Utility.
It also sometimes causes focus changes to a different window when
VoiceOver is launched. The underlying cause of this is that Voiceover
hard-coded (using Chrome and Chromium's bundle IDs) us to automatically
read the web page on an AXLoadComplete event. Webkit fires
AXNewDocumentLoadComplete events rather than AXLoadComplete, so we
adopt that here.

For a much longer explanation and how we debugged this:
https://docs.google.com/document/d/1Rn63cAf1ZVByfh2_ln9rRA388EFM2NVW3wzZo81Bw-c/edit#

AX-Relnotes: Fix a bug where when a page loads, Voiceover focus shifts
to the main content and starts reading it, regardless of whether the
"Automatically speak the webpage" option is checked in VoiceOver Utility.

Bug: 1049320
Change-Id: Icf10fbd09bd17018ffe4d3efe9b656ed790d77c5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2339769Reviewed-by: Aaron Leventhal <aleventhal@chromium.org>
Commit-Queue: Abigail Klein <abigailbklein@google.com>
Cr-Commit-Position: refs/heads/master@{#795490}

content/browser/accessibility/accessibility_event_recorder_mac.mm

@@ -101,8 +101,8 @@ AccessibilityEventRecorderMac::AccessibilityEventRecorderMac(
   AddNotification(@"AXInvalidStatusChanged");
   AddNotification(@"AXLiveRegionChanged");
   AddNotification(@"AXLiveRegionCreated");
-  AddNotification(@"AXLoadComplete");
   AddNotification(@"AXMenuItemSelected");
+  AddNotification(@"AXNewDocumentLoadComplete");
   AddNotification(@"AXRowCollapsed");
   AddNotification(@"AXRowExpanded");
   AddNotification(NSAccessibilityFocusedUIElementChangedNotification);

content/browser/accessibility/browser_accessibility_manager_mac.mm

@@ -69,7 +69,8 @@ enum AXTextEditType {
 NSString* const NSAccessibilityAutocorrectionOccurredNotification =
     @"AXAutocorrectionOccurred";
 NSString* const NSAccessibilityLayoutCompleteNotification = @"AXLayoutComplete";
-NSString* const NSAccessibilityLoadCompleteNotification = @"AXLoadComplete";
+NSString* const NSAccessibilityNewDocumentLoadCompleteNotification =
+    @"AXNewDocumentLoadComplete";
 NSString* const NSAccessibilityInvalidStatusChangedNotification =
     @"AXInvalidStatusChanged";
 NSString* const NSAccessibilityLiveRegionCreatedNotification =
@@ -221,18 +222,23 @@ void BrowserAccessibilityManagerMac::FireGeneratedEvent(
       }
       break;
     case ui::AXEventGenerator::Event::LOAD_COMPLETE:
-      // This notification should only be fired on the top document.
-      // Iframes should use |ax::mojom::Event::kLayoutComplete| to signify that
-      // they have finished loading.
+      // |NSAccessibilityNewDocumentLoadCompleteNotification| should only be
+      // fired on the top document. Iframes should use
+      // |ax::mojom::Event::kLayoutComplete| to signify that they have finished
+      // loading. |NSAccessibilityNewDocumentLoadCompleteNotification| is the
+      // event that Webkit notifies VoiceOver that a page load has completed.
+      // TODO(crbug.com/1049320): Verify in MacOS 10.16 that the "Automatically
+      // speak the webpage" option in the VoiceOver utility is triggered upon
+      // observing this event.
       if (IsRootTree()) {
-        mac_notification = NSAccessibilityLoadCompleteNotification;
+        mac_notification = NSAccessibilityNewDocumentLoadCompleteNotification;
       } else {
         mac_notification = NSAccessibilityLayoutCompleteNotification;
       }
       break;
     case ui::AXEventGenerator::Event::PORTAL_ACTIVATED:
       DCHECK(IsRootTree());
-      mac_notification = NSAccessibilityLoadCompleteNotification;
+      mac_notification = NSAccessibilityLayoutCompleteNotification;
       break;
     case ui::AXEventGenerator::Event::INVALID_STATUS_CHANGED:
       mac_notification = NSAccessibilityInvalidStatusChangedNotification;