[Mac] Refactor the logic of tab dragging out of TabView and into a new helper.

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

Index: chrome/browser/ui/cocoa/tabs/tab_controller.mm
diff --git a/chrome/browser/ui/cocoa/tabs/tab_controller.mm b/chrome/browser/ui/cocoa/tabs/tab_controller.mm
index 2ff7ea9ea18d96219e13cdf077998ca7c3bddedb..4ee4a99d95d959d2b14a514f56fe26881c525dbd 100644
--- a/chrome/browser/ui/cocoa/tabs/tab_controller.mm
+++ b/chrome/browser/ui/cocoa/tabs/tab_controller.mm
@@ -319,4 +319,22 @@ class MenuDelegate : public ui::SimpleMenuModel::Delegate {
   return NO;
 }
 
+// The following methods are invoked from the TabView and are forwarded to the
+// TabStripDragController.
+- (BOOL)tabCanBeDragged:(TabController*)controller {
+  return [[target_ dragController] tabCanBeDragged:controller];
+}
+
+- (void)maybeStartDrag:(NSEvent*)event forTab:(TabController*)tab {
+  [[target_ dragController] maybeStartDrag:event forTab:tab];
+}
+
+- (void)continueDrag:(NSEvent*)event {
+  [[target_ dragController] continueDrag:event];
+}
+
+- (void)endDrag:(NSEvent*)event {
+  [[target_ dragController] endDrag:event];
+}
+
 @end