[Clean] Add unittest target and BrowserCoordinator tests.

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

Index: ios/clean/chrome/browser/browser_coordinator+internal.h
diff --git a/ios/clean/chrome/browser/browser_coordinator+internal.h b/ios/clean/chrome/browser/browser_coordinator+internal.h
index a58d3473362de853a70148ebac426f74819e43f2..43b4a5e6d8a7395c8ee1ea5ca8375bcbf7ccf5a4 100644
--- a/ios/clean/chrome/browser/browser_coordinator+internal.h
+++ b/ios/clean/chrome/browser/browser_coordinator+internal.h
@@ -58,13 +58,17 @@
 // 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