Fix ios_chrome_unittests to crash on DCHECK failure.

ios/chrome/browser/ui/collection_view/collection_view_model_unittest.mm

Index: ios/chrome/browser/ui/collection_view/collection_view_model_unittest.mm
diff --git a/ios/chrome/browser/ui/collection_view/collection_view_model_unittest.mm b/ios/chrome/browser/ui/collection_view/collection_view_model_unittest.mm
index 922bdd37a33512dc88ce0fdc7f8b7a0ce69d8370..09d7543e47f9aa6b1d96626cd4261168b0fd0e91 100644
--- a/ios/chrome/browser/ui/collection_view/collection_view_model_unittest.mm
+++ b/ios/chrome/browser/ui/collection_view/collection_view_model_unittest.mm
@@ -302,9 +302,9 @@ TEST(CollectionViewModelTest, InvalidIndexPath) {
       [[CollectionViewModel alloc] init]);
   [model addSectionWithIdentifier:SectionIdentifierCheese];
 
+  logging::SetLogAssertHandler(&LogSink);

[noyau (Ping after 24h), 2016/12/22 13:17:22]

Suggestions: This would probably be better is handled via a scoped object
resetting the log handler to its previous value when deallocated. Could be done
in a separate CL.

[sdefresne, 2016/12/22 13:28:12]

I agree (there is a similar scoped object in net/test/ but it install a
different assert handler). I'll do it in a followup CL.

   bool out_of_bounds_exception_thrown = false;
   @try {
-    logging::SetLogAssertHandler(&LogSink);
     [model indexInItemTypeForIndexPath:[NSIndexPath indexPathForItem:0
                                                            inSection:0]];
   } @catch (NSException* exception) {
@@ -313,6 +313,7 @@ TEST(CollectionViewModelTest, InvalidIndexPath) {
     }
   }
   EXPECT_TRUE(out_of_bounds_exception_thrown);
+  logging::SetLogAssertHandler(nullptr);
 }
 
 TEST(CollectionViewModelTest, RemoveItems) {