mac: Make dynamic DPI changes work.

content/browser/renderer_host/backing_store_mac.mm

 // Copyright (c) 2012 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 <Cocoa/Cocoa.h>
 
 #include "content/browser/renderer_host/backing_store_mac.h"
 
 #include "base/logging.h"
 #include "base/mac/mac_util.h"
@@ skipping 23 matching lines @@
   if (!cg_layer_) {
     // The view isn't in a window yet.  Use a CGBitmapContext for now.
     cg_bitmap_.reset(CreateCGBitmapContext());
     CGContextScaleCTM(cg_bitmap_, device_scale_factor_, device_scale_factor_);
   }
 }
 
 BackingStoreMac::~BackingStoreMac() {
 }
 
+void BackingStoreMac::ScaleFactorChanged(float device_scale_factor) {
+  if (device_scale_factor == device_scale_factor_)
+    return;
+
+  device_scale_factor_ = device_scale_factor;
+
+  base::mac::ScopedCFTypeRef<CGLayerRef> new_layer(CreateCGLayer());
+  // If we have a layer, copy the old contents. A pixelated flash is better
+  // than a white flash.
+  if (new_layer && cg_layer_) {
+    CGContextRef layer = CGLayerGetContext(new_layer);
+    CGContextDrawLayerAtPoint(layer, CGPointMake(0, 0), cg_layer_);
+  }
+
+  cg_layer_.swap(new_layer);
+  if (!cg_layer_) {
+    // The view isn't in a window yet.  Use a CGBitmapContext for now.
+    cg_bitmap_.reset(CreateCGBitmapContext());
+    CGContextScaleCTM(cg_bitmap_, device_scale_factor_, device_scale_factor_);
+  }
+}
+
 size_t BackingStoreMac::MemorySize() {
   return size().Scale(device_scale_factor_).GetArea() * 4;
 }
 
 void BackingStoreMac::PaintToBackingStore(
     content::RenderProcessHost* process,
     TransportDIB::Id bitmap,
     const gfx::Rect& bitmap_rect,
     const std::vector<gfx::Rect>& copy_rects,
     float scale_factor,
@@ skipping 200 matching lines @@
                                                pixel_size.width(),
                                                pixel_size.height(),
                                                8, pixel_size.width() * 4,
                                                base::mac::GetSystemColorSpace(),
                                                kCGImageAlphaPremultipliedFirst |
                                                    kCGBitmapByteOrder32Host);
   DCHECK(context);
 
   return context;
 }