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

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

Index: chrome/browser/ui/cocoa/tabs/tab_controller_unittest.mm
diff --git a/chrome/browser/ui/cocoa/tabs/tab_controller_unittest.mm b/chrome/browser/ui/cocoa/tabs/tab_controller_unittest.mm
index 4b9fcf7f343d40880f71a5b216d24c79ffddebf0..62781982cad529404cf5f45d3a3c79122e059c9c 100644
--- a/chrome/browser/ui/cocoa/tabs/tab_controller_unittest.mm
+++ b/chrome/browser/ui/cocoa/tabs/tab_controller_unittest.mm
@@ -9,6 +9,7 @@
 #include "chrome/browser/ui/cocoa/cocoa_test_helper.h"
 #import "chrome/browser/ui/cocoa/tabs/tab_controller.h"
 #import "chrome/browser/ui/cocoa/tabs/tab_controller_target.h"
+#import "chrome/browser/ui/cocoa/tabs/tab_strip_drag_controller.h"
 #include "testing/gtest/include/gtest/gtest.h"
 #import "testing/gtest_mac.h"
 #include "testing/platform_test.h"
@@ -19,12 +20,20 @@
  @private
   bool selected_;
   bool closed_;
+  scoped_nsobject<TabStripDragController> dragController_;
 }
 - (bool)selected;
 - (bool)closed;
 @end
 
 @implementation TabControllerTestTarget
+- (id)init {
+  if ((self = [super init])) {
+    dragController_.reset(
+        [[TabStripDragController alloc] initWithTabStripController:nil]);
+  }
+  return self;
+}
 - (bool)selected {
   return selected_;
 }
@@ -67,6 +76,9 @@
   model->AddItem(3, ASCIIToUTF16("Chromium"));
   return model;
 }
+- (id<TabDraggingEventTarget>)dragController {
+  return dragController_.get();
+}
 @end
 
 namespace {