[Clean] Add unittest target and BrowserCoordinator tests.

ios/clean/chrome/browser/browser_coordinator+internal.h

 // Copyright 2016 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.
 
 // ======                        New Architecture                         =====
 // =         This code is only used in the new iOS Chrome architecture.       =
 // ============================================================================
 
 #ifndef IOS_CHROME_BROWSER_BROWSER_COORDINATOR_INTERNAL_H_
 #define IOS_CHROME_BROWSER_BROWSER_COORDINATOR_INTERNAL_H_
@@ skipping 40 matching lines @@
 // the receiver's browserState as the child's browserState.
 - (void)addChildCoordinator:(BrowserCoordinator*)coordinator;
 
 // Removes |coordinator| as a child, relinquishing ownership of it. If
 // |coordinator| isn't a child of the receiver, this method does nothing.
 - (void)removeChildCoordinator:(BrowserCoordinator*)coordinator;
 
 // Methods for adding overlay coordinators.
 
 // Returns YES if the receiver will take |overlayCoordinator| as a child.
-// The default is to return YES only if the receiver has no children.
+// The default is to return YES only if the receiver has no children, if
+// the reciever has a nil -overlayCoordinator, and if |overlayCoordinator|
+// is not already overlaying.
 - (BOOL)canAddOverlayCoordinator:(BrowserCoordinator*)overlayCoordinator;
 
 // Adds |overlayCoordinator| as a child to the receiver, or if it cannot be
 // added, recursively add it to the receiver's child. If a receiver has
 // multiple children and returns YES from -canAddOverlayCoordinator:, it
 // must override this method to determines how the overlay is added.
+// If neither the reciever or any child can add |overlayCoordinator|, then
+// nothing happens.
 - (void)addOverlayCoordinator:(BrowserCoordinator*)overlayCoordinator;
 
 // Removes the current overlay coordinator (if any) as a child from its
 // parent.
 - (void)removeOverlayCoordinator;
 
 @end
 
 #endif  // IOS_CHROME_BROWSER_BROWSER_COORDINATOR_INTERNAL_H_