Bug 633154. When alerts/prompts/confirms are suppressed, just return silently instead of throwing. r=sicking

dom/base/nsGlobalWindow.cpp

@@ -6918,7 +6918,9 @@ nsGlobalWindow::AlertOrConfirm(bool aAlert,
   MOZ_ASSERT(IsOuterWindow());
 
   if (!AreDialogsEnabled()) {
-    aError.Throw(NS_ERROR_NOT_AVAILABLE);
+    // Just silently return.  In the case of alert(), the return value is
+    // ignored.  In the case of confirm(), returning false is the same thing as
+    // would happen if the user cancels.
     return false;
   }
 
@@ -7058,7 +7060,7 @@ nsGlobalWindow::PromptOuter(const nsAString& aMessage, const nsAString& aInitial
   SetDOMStringToNull(aReturn);
 
   if (!AreDialogsEnabled()) {
-    aError.Throw(NS_ERROR_NOT_AVAILABLE);
+    // Return null, as if the user just canceled the prompt.
     return;
   }
 
@@ -7496,6 +7498,8 @@ nsGlobalWindow::PrintOuter(ErrorResult& aError)
   }
 
   if (!AreDialogsEnabled()) {
+    // We probably want to keep throwing here; silently doing nothing is a bit
+    // weird given the typical use cases of print().
     aError.Throw(NS_ERROR_NOT_AVAILABLE);
     return;
   }
@@ -9553,6 +9557,8 @@ nsGlobalWindow::ShowModalDialogOuter(const nsAString& aUrl, nsIVariant* aArgumen
   EnsureReflowFlushAndPaint();
 
   if (!AreDialogsEnabled()) {
+    // We probably want to keep throwing here; silently doing nothing is a bit
+    // weird given the typical use cases of showModalDialog().
     aError.Throw(NS_ERROR_NOT_AVAILABLE);
     return nullptr;
   }