Simplify platform_canvas.h by recognizing that PlatformCanvas does not actually extend

skia/ext/bitmap_platform_device_mac.cc

Index: skia/ext/bitmap_platform_device_mac.cc
===================================================================
--- skia/ext/bitmap_platform_device_mac.cc	(revision 165342)
+++ skia/ext/bitmap_platform_device_mac.cc	(working copy)
@@ -255,6 +255,21 @@
   return bitmap_device;
 }
 
+// PlatformCanvas impl
+
+SkCanvas* CreatePlatformCanvas(CGContextRef ctx, int width, int height,
+                               bool is_opaque, OnFailureType failureType) {
+  SkDevice* dev = BitmapPlatformDevice::Create(ctx, width, height, is_opaque);
+  return CreateCanvas(dev, failureType);
+}
+
+SkCanvas* CreatePlatformCanvas(int width, int height, bool is_opaque,
+                               uint8_t* data, OnFailureType failureType) {
+  SkDevice* dev = BitmapPlatformDevice::CreateWithData(data, width, height,
+                                                       is_opaque);
+  return CreateCanvas(dev, failureType);
+}
+
 // Port of PlatformBitmap to mac
 
 PlatformBitmap::~PlatformBitmap() {