VR: fix AlertDialog showing up in 2D

VrUiWidgetFactory should be initialized with VrModalDialogManager
instead of ModalDialogManager.

The issue appeared after this CL:
https://chromium-review.googlesource.com/c/chromium/src/+/1136856

Bug: 866511
Change-Id: I0cc8fa0a0bf951415d85a44c5fe48c6fa20999a7
Reviewed-on: https://chromium-review.googlesource.com/1147214
Commit-Queue: Amirhossein Simjour <asimjour@chromium.org>
Reviewed-by: Aldo Culquicondor <acondor@chromium.org>
Cr-Commit-Position: refs/heads/master@{#577501}

chrome/android/java/src/org/chromium/chrome/browser/vr/VrShellImpl.java

@@ -164,12 +164,6 @@ public class VrShellImpl
             mActivity.getFindToolbarManager().hideToolbar();
         }
 
-        if (ChromeFeatureList.isEnabled(ChromeFeatureList.VR_BROWSING_NATIVE_ANDROID_UI)) {
-            mNonVrUiWidgetFactory = UiWidgetFactory.getInstance();
-            UiWidgetFactory.setInstance(
-                    new VrUiWidgetFactory(this, mActivity.getModalDialogManager()));
-        }
-
         // This overrides the default intent created by GVR to return to Chrome when the DON flow
         // is triggered by resuming the GvrLayout, which is the usual way Daydream apps enter VR.
         // See VrShellDelegate#getEnterVrPendingIntent for why we need to do this.
@@ -200,6 +194,13 @@ public class VrShellImpl
 
         if (mVrBrowsingEnabled) injectVrHostedUiView();
 
+        // This has to happen after VrModalDialogManager is created.
+        if (ChromeFeatureList.isEnabled(ChromeFeatureList.VR_BROWSING_NATIVE_ANDROID_UI)) {
+            mNonVrUiWidgetFactory = UiWidgetFactory.getInstance();
+            UiWidgetFactory.setInstance(
+                    new VrUiWidgetFactory(this, mActivity.getModalDialogManager()));
+        }
+
         mTabRedirectHandler = new TabRedirectHandler(mActivity) {
             @Override
             public boolean shouldStayInChrome(boolean hasExternalProtocol) {