[ios] Adds ScopedBlockSwizzler.

ios/chrome/test/base/scoped_block_swizzler.mm

Index: ios/chrome/test/base/scoped_block_swizzler.mm
diff --git a/ios/chrome/test/base/scoped_block_swizzler.mm b/ios/chrome/test/base/scoped_block_swizzler.mm
new file mode 100644
index 0000000000000000000000000000000000000000..312de59466c7db44f6b27280d89b845baf38a0cb
--- /dev/null
+++ b/ios/chrome/test/base/scoped_block_swizzler.mm
@@ -0,0 +1,24 @@
+// Copyright 2013 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/test/base/scoped_block_swizzler.h"
+
+#include "base/logging.h"
+
+ScopedBlockSwizzler::ScopedBlockSwizzler(Class target, SEL selector, id block) {
+  method_ = class_getInstanceMethod(target, selector);
+  if (!method_) {
+    // Try swizzling a class method instead.
+    method_ = class_getClassMethod(target, selector);
+  }
+  DCHECK(method_);
+
+  IMP block_imp = imp_implementationWithBlock(block);

[sdefresne, 2016/12/05 15:56:00]

The implementation requires that the block outlives the ScopedBlockSwizzler.
Maybe we can copy the block and use a ScopedBlock to keep it alive. WDYT?

[rohitrao (ping after 24h), 2016/12/05 15:58:11]

Seems like a good idea, although then we'd potentially be changing the lifetime
of objects under test.  If the block captures an object and is autoreleased
after the test ends, we may unexpectedly introduce crashes (generally due to
bugs in the test).

I'll file a bug for this and we can follow up.

+  original_imp_ = method_setImplementation(method_, block_imp);
+}
+
+ScopedBlockSwizzler::~ScopedBlockSwizzler() {
+  IMP block_imp = method_setImplementation(method_, original_imp_);
+  DCHECK(imp_removeBlock(block_imp));
+}