[Mac][Material Design] Update Omnibox dropdown on Mac to MD

chrome/browser/ui/cocoa/omnibox/omnibox_popup_separator_view.mm

 // Copyright 2013 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/ui/cocoa/omnibox/omnibox_popup_separator_view.h"
 
+#import "base/mac/scoped_nsobject.h"
 #include "grit/theme_resources.h"
 #import "ui/base/cocoa/nsview_additions.h"
 #include "ui/base/resource/resource_bundle.h"
 
 @implementation OmniboxPopupTopSeparatorView
 
 + (CGFloat)preferredHeight {
   return 1;
 }
 
 - (void)drawRect:(NSRect)rect {
   NSRect separatorRect = [self bounds];
   separatorRect.size.height = [self cr_lineWidth];
   [[self strokeColor] set];
   NSRectFillUsingOperation(separatorRect, NSCompositeSourceOver);
 }
 
 @end
 
 @implementation OmniboxPopupBottomSeparatorView
 
 + (CGFloat)preferredHeight {
   ResourceBundle& rb = ResourceBundle::GetSharedInstance();
   NSImage* shadowImage =
       rb.GetNativeImageNamed(IDR_OVERLAY_DROP_SHADOW).ToNSImage();
   return [shadowImage size].height;
 }
 
+- (instancetype)initWithFrame:(NSRect)frame forDarkTheme:(BOOL)isDarkTheme {
+  if ((self = [self initWithFrame:frame])) {
+    isDarkTheme_ = isDarkTheme;
+    // For dark themes the OmniboxPopupBottomSeparatorView will render a shadow
+    // rather than blit a bitmap. Shadows are expensive to draw so use a layer
+    // to cache the result.
+    if (isDarkTheme_) {
+      [self setWantsLayer:YES];
+    }
+  }
+  return self;
+}
+
 - (void)drawRect:(NSRect)rect {
-  ResourceBundle& rb = ResourceBundle::GetSharedInstance();
   NSRect bounds = [self bounds];
 
+  if (isDarkTheme_) {
+    // There's an image for the shadow the Omnibox casts, but this shadow is
+    // an opaque mix of white and black, which makes it look strange against a
+    // dark NTP page. For dark mode, draw the shadow in code instead so that
+    // it has some transparency.
+    base::scoped_nsobject<NSShadow> shadow([[NSShadow alloc] init]);
+    [shadow setShadowBlurRadius:8];
+    [shadow setShadowColor:[NSColor blackColor]];
+    [shadow set];
+
+    // Fill a rect that's out of view to get just the shadow it casts.
+    [[NSColor blackColor] set];
+    NSRectFill(NSMakeRect(-3, NSMaxY(bounds), NSWidth(bounds) + 6, 5));
+    return;
+  }
+
   // Draw the shadow.
+  ResourceBundle& rb = ResourceBundle::GetSharedInstance();
   NSImage* shadowImage =
       rb.GetNativeImageNamed(IDR_OVERLAY_DROP_SHADOW).ToNSImage();
   [shadowImage drawInRect:bounds
                  fromRect:NSZeroRect
                 operation:NSCompositeSourceOver
                  fraction:1.0];
 }
 
 @end