Extract TabModelObservers from tab_model.mm to its own file.

ios/chrome/browser/tabs/tab_model_observers.mm

Index: ios/chrome/browser/tabs/tab_model_observers.mm
diff --git a/ios/chrome/browser/tabs/tab_model_observers.mm b/ios/chrome/browser/tabs/tab_model_observers.mm
new file mode 100644
index 0000000000000000000000000000000000000000..fd86c711e5c924327bb9623784f9a9c559896420
--- /dev/null
+++ b/ios/chrome/browser/tabs/tab_model_observers.mm
@@ -0,0 +1,17 @@
+// 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.
+
+#import "ios/chrome/browser/tabs/tab_model_observers.h"
+
+#if !defined(__has_feature) || !__has_feature(objc_arc)
+#error "This file requires ARC support."
+#endif
+
+@implementation TabModelObservers
+
++ (instancetype)observers {

[rohitrao (ping after 24h), 2017/02/13 19:24:28]

Was this not necessary in the original implementation?  Why have you added it
now?

[sdefresne, 2017/02/14 10:11:31]

This is just a convenience method, not necessary but allow for shorter lines by
replacing:

  TabModelObserver* observers = [TabModelObservers
      observersWithProtocol:@protocol(TabModelObserver)];

with:

  TabModelObserver* observers = [TabModelObservers observers];

+  return [self observersWithProtocol:@protocol(TabModelObserver)];
+}
+
+@end