Fix event targeting for overlay scrollbar thumbs (in native UI).

ui/views/controls/scrollbar/cocoa_scroll_bar.mm

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #import "ui/views/controls/scrollbar/cocoa_scroll_bar.h"
 
 #import "base/mac/sdk_forward_declarations.h"
 #include "third_party/skia/include/core/SkColor.h"
 #include "third_party/skia/include/effects/SkGradientShader.h"
 #include "ui/compositor/layer.h"
@@ skipping 143 matching lines @@
   // the scrollview. The thumb will be set back to its hover or normal state
   // when the mouse is released.
   if (GetState() != CustomButton::STATE_PRESSED)
     SetState(CustomButton::STATE_NORMAL);
 }
 
 //////////////////////////////////////////////////////////////////
 // CocoaScrollBar class
 
 CocoaScrollBar::CocoaScrollBar(bool horizontal)
-    : BaseScrollBar(horizontal, new CocoaScrollBarThumb(this)),
+    : BaseScrollBar(horizontal),
       hide_scrollbar_timer_(
           FROM_HERE,
           base::TimeDelta::FromMilliseconds(kScrollbarHideTimeoutMs),
           base::Bind(&CocoaScrollBar::HideScrollbar, base::Unretained(this)),
           false),
       thickness_animation_(this),
       last_contents_scroll_offset_(0),
       is_expanded_(false),
       did_start_dragging_(false) {
+  SetThumb(new CocoaScrollBarThumb(this));
   bridge_.reset([[ViewsScrollbarBridge alloc] initWithDelegate:this]);
   scroller_style_ = [ViewsScrollbarBridge getPreferredScrollerStyle];
 
   thickness_animation_.SetSlideDuration(kExpandDurationMs);
 
   SetPaintToLayer(true);
   has_scrolltrack_ = scroller_style_ == NSScrollerStyleLegacy;
   layer()->SetOpacity(scroller_style_ == NSScrollerStyleOverlay ? 0.0f : 1.0f);
 }
 
@@ skipping 351 matching lines @@
 CocoaScrollBarThumb* CocoaScrollBar::GetCocoaScrollBarThumb() const {
   return static_cast<CocoaScrollBarThumb*>(GetThumb());
 }
 
 // static
 base::Timer* BaseScrollBar::GetHideTimerForTest(BaseScrollBar* scroll_bar) {
   return &static_cast<CocoaScrollBar*>(scroll_bar)->hide_scrollbar_timer_;
 }
 
 }  // namespace views