[Mac] Make Omnibox a little bit transparent.

chrome/browser/autocomplete/autocomplete_popup_view_mac.mm

 // Copyright (c) 2009 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.
 
 #include "chrome/browser/autocomplete/autocomplete_popup_view_mac.h"
 
 #include "app/gfx/text_elider.h"
 #include "base/sys_string_conversions.h"
 #include "base/gfx/rect.h"
 #include "chrome/browser/autocomplete/autocomplete_edit.h"
@@ skipping 12 matching lines @@
 // by the font.
 const int kCellHeightAdjust = 7.0;
 
 // How to round off the popup's corners.  Goal is to match star and go
 // buttons.
 const CGFloat kPopupRoundingRadius = 3.5;
 
 // Gap between the field and the popup.
 const CGFloat kPopupFieldGap = 2.0;
 
+// How opaque the popup window should be.  This matches Windows (see
+// autocomplete_popup_contents_view.cc, kGlassPopupTransparency).
+const CGFloat kPopupAlpha = 240.0 / 255.0;
+
 // How much space to leave for the left and right margins.
 const CGFloat kLeftRightMargin = 8.0;
 
 // How far to offset the text column from the left.
 const CGFloat kTextXOffset = 33.0;
 
 // Animation duration when animating the popup window smaller.
 const float kShrinkAnimationDuration = 0.1;
 
 // Maximum fraction of the popup width that can be used to display match
 // contents.
 const float kMaxMatchContentsWidth = 0.7;
 
 // NSEvent -buttonNumber for middle mouse button.
 const static NSInteger kMiddleButtonNumber(2);
 
 // Background colors for different states of the popup elements.
 NSColor* BackgroundColor() {
-  return [NSColor controlBackgroundColor];
+  return [[NSColor controlBackgroundColor] colorWithAlphaComponent:kPopupAlpha];
 }
 NSColor* SelectedBackgroundColor() {
-  return [NSColor selectedControlColor];
+  return [[NSColor selectedControlColor] colorWithAlphaComponent:kPopupAlpha];
 }
 NSColor* HoveredBackgroundColor() {
-  return [NSColor controlColor];
+  return [[NSColor controlHighlightColor] colorWithAlphaComponent:kPopupAlpha];
 }
 
 // TODO(shess): These are totally unprincipled.  I experimented with
 // +controlTextColor and the like, but found myself wondering whether
 // that was really appropriate.  Circle back after consulting with
 // someone more knowledgeable about the ins and outs of this.
 static const NSColor* ContentTextColor() {
   return [NSColor blackColor];
 }
 static const NSColor* URLTextColor() {
@@ skipping 624 matching lines @@
   DCHECK(popup_view_);
 
   // TODO(shess): UpdatePopupAppearance() is called frequently, so it
   // should be really cheap, but in this case we could probably make
   // things even cheaper by refactoring between the popup-placement
   // code and the matrix-population code.
   popup_view_->UpdatePopupAppearance();
 }
 
 @end