[Mac]Implement ViewID support. (third approach)

chrome/browser/cocoa/tab_strip_controller.h

Index: chrome/browser/cocoa/tab_strip_controller.h
diff --git a/chrome/browser/cocoa/tab_strip_controller.h b/chrome/browser/cocoa/tab_strip_controller.h
index 5caf3817e91600836d08c8c1789acac74ed3814a..ace64ba7374fc3b2101a792b44465a164c2417f2 100644
--- a/chrome/browser/cocoa/tab_strip_controller.h
+++ b/chrome/browser/cocoa/tab_strip_controller.h
@@ -157,6 +157,11 @@ class ToolbarModel;
 // Return the view at a given index.
 - (NSView*)viewAtIndex:(NSUInteger)index;
 
+// Return the number of tab views in the tab strip. It's same as number of tabs
+// in the model, except when a tab is closing, which will be counted in views
+// count, but no longer in the model.
+- (NSUInteger)viewsCount;
+
 // Set the placeholder for a dragged tab, allowing the |frame| and |strechiness|
 // to be specified. This causes this tab to be rendered in an arbitrary position
 - (void)insertPlaceholderForTab:(TabView*)tab