WebLayer: Modify assert during modal dialog dismissal.

Bug: 1125860
Change-Id: I1aa892bad4b66a8518076244025f6fbafcf9b518
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2399403Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Evan Stade <estade@chromium.org>
Cr-Commit-Position: refs/heads/master@{#805332}

weblayer/browser/java/org/chromium/weblayer_private/BrowserViewController.java

@@ -299,9 +299,11 @@ public final class BrowserViewController
     private void onDialogVisibilityChanged(boolean showing) {
         if (WebLayerFactoryImpl.getClientMajorVersion() < 82) return;
 
-        // This should never be called when |mTab| is null.
-        assert mTab != null;
         if (mModalDialogManager.getCurrentType() == ModalDialogType.TAB) {
+            // This shouldn't be called when |mTab| is null and the modal dialog type is TAB. OTOH,
+            // when an app-modal is displayed for a javascript dialog, this method can be called
+            // after the tab is destroyed.
+            assert mTab != null;
             try {
                 mTab.getClient().onTabModalStateChanged(showing);
             } catch (RemoteException e) {