Plugin VM installer: Notify AX system when text changes

This CL updates the Plugin VM installer so that ChromeVox users can get
updates to the labels (title, message, download progress and time left).

Bug: 1021414
Change-Id: I4e80b95ceb232bb68327ee296da07054a2464aea
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1935295Reviewed-by: Chris Hall <chrishall@chromium.org>
Commit-Queue: Timothy Loh <timloh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#719039}

chrome/browser/ui/views/plugin_vm/plugin_vm_launcher_view.cc

@@ -17,6 +17,7 @@
 #include "chrome/grit/chrome_unscaled_resources.h"
 #include "chrome/grit/generated_resources.h"
 #include "content/public/browser/browser_thread.h"
+#include "ui/accessibility/ax_enums.mojom.h"
 #include "ui/base/l10n/l10n_util.h"
 #include "ui/base/l10n/time_format.h"
 #include "ui/base/resource/resource_bundle.h"
@@ -275,6 +276,8 @@ void PluginVmLauncherView::OnDownloadProgressUpdated(
 
   download_progress_message_label_->SetText(
       GetDownloadProgressMessage(bytes_downloaded, content_length));
+  download_progress_message_label_->NotifyAccessibilityEvent(
+      ax::mojom::Event::kTextChanged, true);
   UpdateOperationProgress(bytes_downloaded, content_length, elapsed_time);
 }
 
@@ -495,16 +498,22 @@ void PluginVmLauncherView::UpdateOperationProgress(
   time_left_message_label_->SetText(
       ui::TimeFormat::Simple(ui::TimeFormat::FORMAT_REMAINING,
                              ui::TimeFormat::LENGTH_SHORT, remaining));
+  time_left_message_label_->NotifyAccessibilityEvent(
+      ax::mojom::Event::kTextChanged, true);
 }
 
 void PluginVmLauncherView::SetBigMessageLabel() {
   big_message_label_->SetText(GetBigMessage());
   big_message_label_->SetVisible(true);
+  big_message_label_->NotifyAccessibilityEvent(ax::mojom::Event::kTextChanged,
+                                               true);
 }
 
 void PluginVmLauncherView::SetMessageLabel() {
   message_label_->SetText(GetMessage());
   message_label_->SetVisible(true);
+  message_label_->NotifyAccessibilityEvent(ax::mojom::Event::kTextChanged,
+                                           true);
 }
 
 void PluginVmLauncherView::SetBigImage() {