Bug 1693434 - Report uncaught Objective C exceptions. r=spohl

NSSetUncaughtExceptionHandler installs a handler which is called just before we
would crash anyway, I think. So this handler lets us collect information about
exceptions that are not even caught by the native event loop.

Differential Revision: https://phabricator.services.mozilla.com/D105561

widget/cocoa/nsAppShell.mm

@@ -122,8 +122,18 @@ extern int32_t gXULModalLevel;
 
 static bool gAppShellMethodsSwizzled = false;
 
+void OnUncaughtException(NSException* aException) {
+  nsObjCExceptionLog(aException);
+  MOZ_CRASH("Uncaught Objective C exception from NSSetUncaughtExceptionHandler");
+}
+
 @implementation GeckoNSApplication
 
+// Load is called very early during startup, when the Objective C runtime loads this class.
++ (void)load {
+  NSSetUncaughtExceptionHandler(OnUncaughtException);
+}
+
 - (void)sendEvent:(NSEvent*)anEvent {
   mozilla::BackgroundHangMonitor().NotifyActivity();