[Material][Mac] Fix tabstrip and avatar button hierarchy

chrome/browser/ui/cocoa/tabs/tab_window_controller.mm

Index: chrome/browser/ui/cocoa/tabs/tab_window_controller.mm
diff --git a/chrome/browser/ui/cocoa/tabs/tab_window_controller.mm b/chrome/browser/ui/cocoa/tabs/tab_window_controller.mm
index 3f6e67d13334f4cbbda03034a9c20faea796a761..85b91a7a6c7ed4367397c747cec31dbc4b1e0f55 100644
--- a/chrome/browser/ui/cocoa/tabs/tab_window_controller.mm
+++ b/chrome/browser/ui/cocoa/tabs/tab_window_controller.mm
@@ -182,15 +182,15 @@
     // Return the original window's tab strip view and content view to their
     // places. The TabStripView always needs to be in front of the window's
     // content view and therefore it should always be added after the content
-    // view is set. It needs to be positioned at the bottom of the view
-    // hierarchy to ensure that it does not overlap the avatar button icon.
+    // view is set. It needs to be positioned below the avatar button to ensure
+    // that its overlay will not overlap it.
     [[window contentView] addSubview:originalContentView_
                           positioned:NSWindowBelow
                           relativeTo:nil];
     originalContentView_.frame = [[window contentView] bounds];
     [[window contentView] addSubview:[self tabStripView]
                           positioned:NSWindowBelow
-                          relativeTo:nil];
+                          relativeTo:[self avatarView]];
     [[self tabStripView] setInATabDraggingOverlayWindow:NO];
     [[window contentView] updateTrackingAreas];
 
@@ -315,6 +315,10 @@
   return 0;
 }
 
+- (NSView*)avatarView {
+  return nil;
+}
+
 - (NSString*)activeTabTitle {
   // subclass must implement
   NOTIMPLEMENTED();