[Mac] Fix unit_tests build in 10.12 SDK.

chrome/browser/mac/exception_processor_unittest.mm

 // Copyright 2016 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 "chrome/browser/mac/exception_processor.h"
 
 #import <Cocoa/Cocoa.h>
 #include <stddef.h>
 #include <sys/wait.h>
 
@@ skipping 77 matching lines @@
   EXPECT_TRUE(histograms[0]->HasConstructionArguments(1,
                                                       kUnknownNSException,
                                                       kUnknownNSException + 1));
   EXPECT_EQ(4, samples->GetCount(kUnknownNSException));
 }
 
 void RaiseExceptionInRunLoop() {
   CFRunLoopRef run_loop = CFRunLoopGetCurrent();
 
   CFRunLoopPerformBlock(run_loop, kCFRunLoopCommonModes, ^{
-    [NSException raise:@"ThrowExceptionInRunLoop" format:nil];
+    [NSException raise:@"ThrowExceptionInRunLoop" format:@""];
   });
   CFRunLoopPerformBlock(run_loop, kCFRunLoopCommonModes, ^{
     CFRunLoopStop(run_loop);
   });
   CFRunLoopRun();
 }
 
 void ThrowExceptionInRunLoop() {
   base::mac::DisableOSCrashDumps();
   chrome::InstallObjcExceptionPreprocessor();
@@ skipping 13 matching lines @@
                "Terminating from Objective-C exception:.*");
 }
 
 void ThrowAndCatchExceptionInRunLoop() {
   base::mac::DisableOSCrashDumps();
   chrome::InstallObjcExceptionPreprocessor();
 
   CFRunLoopRef run_loop = CFRunLoopGetCurrent();
   CFRunLoopPerformBlock(run_loop, kCFRunLoopCommonModes, ^{
     @try {
-      [NSException raise:@"ObjcExceptionPreprocessCaught" format:nil];
+      [NSException raise:@"ObjcExceptionPreprocessCaught" format:@""];
     } @catch (id exception) {
     }
   });
 
   CFRunLoopPerformBlock(run_loop, kCFRunLoopCommonModes, ^{
     CFRunLoopStop(run_loop);
   });
 
   CFRunLoopRun();
 
@@ skipping 30 matching lines @@
 TEST(ExceptionProcessorTest, ThrowExceptionInRunLoopWithoutProcessor) {
   ::testing::FLAGS_gtest_death_test_style = "threadsafe";
   EXPECT_EXIT(ThrowExceptionInRunLoopWithoutProcessor(),
               [](int exit_code) -> bool {
                 return WEXITSTATUS(exit_code) == 0;
               },
               ".*TEST PASS.*");
 }
 
 }  // namespace chrome