Create MetricsRecorder protocol.

Create a MetricsRecorder protocol that allows implementors to record
a metric for a given NSInvocation. Objects implementing this protocol
will be able to register with the dispatcher to be called when a
relevant selector is invoked on the dispatcher.

Bug: none
Change-Id: Icb0074f6d3857ea9b7c0043c052e5e35dc6598ec
Reviewed-on: https://chromium-review.googlesource.com/578305Reviewed-by: Mark Cogan <marq@chromium.org>
Commit-Queue: Gregory Chatzinoff <gchatz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#488710}

ios/shared/chrome/browser/ui/metrics/BUILD.gn

+# Copyright 2017 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.
+
+source_set("metrics") {
+  sources = [
+    "metrics_recorder.h",
+  ]
+
+  configs += [ "//build/config/compiler:enable_arc" ]
+}

ios/shared/chrome/browser/ui/metrics/metrics_recorder.h

+// Copyright 2017 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.
+
+#ifndef IOS_SHARED_CHROME_BROWSER_UI_METRICS_METRICS_RECORDER_H_
+#define IOS_SHARED_CHROME_BROWSER_UI_METRICS_METRICS_RECORDER_H_
+
+#import <Foundation/Foundation.h>
+
+// MetricsRecorder defines a common interface for objects that record metrics
+// for a given NSInvocation object.
+@protocol MetricsRecorder<NSObject>
+
+// Records metrics for |anInvocation|.
+- (void)recordMetricForInvocation:(NSInvocation*)anInvocation;
+
+@end
+
+#endif  // IOS_SHARED_CHROME_BROWSER_UI_METRICS_METRICS_RECORDER_H_