Enable -Wundeclared-selector on ios.

ios/chrome/browser/snapshots/snapshot_cache_unittest.mm

 // Copyright 2012 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 "ios/chrome/browser/snapshots/snapshot_cache.h"
 
 #import <Foundation/Foundation.h>
 
 #include "base/files/file_path.h"
 #include "base/files/file_util.h"
@@ skipping 181 matching lines @@
   }
 
   const char* GetPixelData(CGImageRef cgImage) {
     CFDataRef data = CGDataProviderCopyData(CGImageGetDataProvider(cgImage));
     return reinterpret_cast<const char*>(CFDataGetBytePtr(data));
   }
 
   void TriggerMemoryWarning() {
     // _performMemoryWarning is a private API and must not be compiled into
     // official builds.
+#pragma clang diagnostic push
+#pragma clang diagnostic ignored "-Wundeclared-selector"
     [[UIApplication sharedApplication]
         performSelector:@selector(_performMemoryWarning)];
+#pragma clang diagnostic pop
   }
 
   web::TestWebThreadBundle thread_bundle_;
   base::scoped_nsobject<SnapshotCache> snapshotCache_;
   base::scoped_nsobject<NSMutableArray> testSessions_;
   base::scoped_nsobject<NSMutableArray> testImages_;
 };
 
 // This test simply put all the snapshots in the cache and then gets them back
 // As the snapshots are kept in memory, the same pointer can be retrieved.
@@ skipping 352 matching lines @@
   EXPECT_TRUE(base::PathExists(retinaFile));
 
   // Delete the image.
   [cache removeImageWithSessionID:kSession];
   FlushRunLoops();  // ensure the file is removed.
 
   EXPECT_FALSE(base::PathExists(retinaFile));
 }
 
 }  // namespace