[Mac] Allow NSExceptions in certain cases.

base/mac/scoped_nsexception_enabler.mm

Index: base/mac/scoped_nsexception_enabler.mm
diff --git a/base/mac/scoped_nsexception_enabler.mm b/base/mac/scoped_nsexception_enabler.mm
new file mode 100644
index 0000000000000000000000000000000000000000..e79dcd9981fbf3388d2b6c28df60321d0f1a9386
--- /dev/null
+++ b/base/mac/scoped_nsexception_enabler.mm
@@ -0,0 +1,44 @@
+// Copyright (c) 2011 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#import "base/mac/scoped_nsexception_enabler.h"
+
+#import "base/lazy_instance.h"
+#import "base/threading/thread_local.h"
+
+// To make the |g_exceptionsAllowed| declaration readable.
+using base::LazyInstance;
+using base::LeakyLazyInstanceTraits;
+using base::ThreadLocalBoolean;
+
+namespace {
+
+// Whether to allow NSExceptions to be raised on the current thread.
+LazyInstance<ThreadLocalBoolean, LeakyLazyInstanceTraits<ThreadLocalBoolean> >
+    g_exceptionsAllowed(base::LINKER_INITIALIZED);
+
+}  // namespace
+
+namespace base {
+namespace mac {
+
+bool GetNSExceptionsAllowed() {
+  return g_exceptionsAllowed.Get().Get();
+}
+
+void SetNSExceptionsAllowed(bool allowed) {
+  return g_exceptionsAllowed.Get().Set(allowed);
+}
+
+ScopedNSExceptionEnabler::ScopedNSExceptionEnabler(bool enable) {
+  was_enabled_ = GetNSExceptionsAllowed();
+  SetNSExceptionsAllowed(was_enabled_ || enable);
+}
+
+ScopedNSExceptionEnabler::~ScopedNSExceptionEnabler() {
+  SetNSExceptionsAllowed(was_enabled_);
+}
+
+}  // namespace mac
+}  // namespace base