Refactor the PlatformContext layer to have only one class.

skia/ext/platform_device_mac.h

Index: skia/ext/platform_device_mac.h
diff --git a/skia/ext/platform_device_mac.h b/skia/ext/platform_device_mac.h
index 2005e89ae0686b92b28579a8ab783c6a3d58221f..97efd7c1b2f497208083f56923f79fc2a86c8191 100644
--- a/skia/ext/platform_device_mac.h
+++ b/skia/ext/platform_device_mac.h
@@ -20,8 +20,10 @@ namespace skia {
 // functions.
 // This class is abstract and must be subclassed. It provides the basic
 // interface to implement it either with or without a bitmap backend.
-class PlatformDeviceMac : public SkDevice {
+class PlatformDevice : public SkDevice {
  public:
+  typedef CGContextRef PlatformSurface;
+
   // The CGContext that corresponds to the bitmap, used for CoreGraphics
   // operations drawing into the bitmap. This is possibly heavyweight, so it
   // should exist only during one pass of rendering.
@@ -52,7 +54,7 @@ class PlatformDeviceMac : public SkDevice {
 
  protected:
   // Forwards |bitmap| to SkDevice's constructor.
-  PlatformDeviceMac(const SkBitmap& bitmap);
+  PlatformDevice(const SkBitmap& bitmap);
 
   // Loads the specified Skia transform into the device context
   static void LoadTransformToCGContext(CGContextRef context,