[Mac]Implement ViewID support.

chrome/browser/cocoa/side_tab_strip_view.mm

 // Copyright (c) 2010 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 "chrome/browser/cocoa/side_tab_strip_view.h"
 
 #include "base/scoped_nsobject.h"
+#import "chrome/browser/cocoa/view_id_util.h"
 #import "third_party/GTM/AppKit/GTMNSColor+Luminance.h"
 
 @implementation SideTabStripView
 
 - (void)drawBorder:(NSRect)bounds {
   // Draw a border on the right side.
   NSRect borderRect, contentRect;
   NSDivideRect(bounds, &borderRect, &contentRect, 1, NSMaxXEdge);
   [[NSColor colorWithCalibratedWhite:0.0 alpha:0.2] set];
   NSRectFillUsingOperation(borderRect, NSCompositeSourceOver);
@@ skipping 15 matching lines @@
   scoped_nsobject<NSGradient> gradient(
       [[NSGradient alloc] initWithStartingColor:aColor endingColor:bColor]);
 
   NSRect gradientRect = [self bounds];
   [gradient drawInRect:gradientRect angle:270.0];
 
   // Draw borders and any drop feedback.
   [super drawRect:rect];
 }
 
+// Tag is used solely for ViewID. Override to prevent changing of it.
+- (NSInteger)tag {
+  return view_id_util::ViewIDToTag(VIEW_ID_TAB_STRIP);
+}
+
 @end