[Mac]Implement ViewID support.

chrome/browser/cocoa/tab_controller.mm

Index: chrome/browser/cocoa/tab_controller.mm
diff --git a/chrome/browser/cocoa/tab_controller.mm b/chrome/browser/cocoa/tab_controller.mm
index 50eb2688b07123aed74a9c712609d1bb4c7784f0..fbf57374332855594227682fd8878dc5faa93002 100644
--- a/chrome/browser/cocoa/tab_controller.mm
+++ b/chrome/browser/cocoa/tab_controller.mm
@@ -314,4 +314,16 @@ class MenuDelegate : public menus::SimpleMenuModel::Delegate {
   return NO;
 }
 
+- (ViewID)viewID {
+  NSInteger total = 0;
+  NSInteger index = [[self target] indexForTabController:self inTotal:&total];
+  DCHECK(index >= 0 && index < total);
+
+  if (index >= 0 && index <= (VIEW_ID_TAB_9 - VIEW_ID_TAB_0))
+    return static_cast<ViewID>(index + VIEW_ID_TAB_0);
+  else if (index == total - 1)
+    return VIEW_ID_TAB_LAST;
+  return VIEW_ID_NONE;
+}
+
 @end