[Mac] Fix unit_tests build in 10.12 SDK.

chrome/browser/mac/exception_processor_unittest.mm

Index: chrome/browser/mac/exception_processor_unittest.mm
diff --git a/chrome/browser/mac/exception_processor_unittest.mm b/chrome/browser/mac/exception_processor_unittest.mm
index 74c1128fe65a6bc72aedd0ec1d3d07d5a5465110..0e8460be058769ea5003adcc09c4ff5dc24040f1 100644
--- a/chrome/browser/mac/exception_processor_unittest.mm
+++ b/chrome/browser/mac/exception_processor_unittest.mm
@@ -95,7 +95,7 @@ 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);
@@ -129,7 +129,7 @@ void ThrowAndCatchExceptionInRunLoop() {
   CFRunLoopRef run_loop = CFRunLoopGetCurrent();
   CFRunLoopPerformBlock(run_loop, kCFRunLoopCommonModes, ^{
     @try {
-      [NSException raise:@"ObjcExceptionPreprocessCaught" format:nil];
+      [NSException raise:@"ObjcExceptionPreprocessCaught" format:@""];
     } @catch (id exception) {
     }
   });