Optionally clear PlatformCanvas instances.

skia/ext/bitmap_platform_device_mac.h

Index: skia/ext/bitmap_platform_device_mac.h
===================================================================
--- skia/ext/bitmap_platform_device_mac.h	(revision 127021)
+++ skia/ext/bitmap_platform_device_mac.h	(working copy)
@@ -28,12 +28,22 @@
 // DEVICE'S PIXEL DATA TO ANOTHER BITMAP, make sure you copy instead.
 class BitmapPlatformDevice : public PlatformDevice, public SkDevice {
  public:
-  // |context| may be NULL.
+  // Creates a BitmapPlatformDevice instance. |is_opaque| should be set if the
+  // caller knows the bitmap will be completely opaque and allows some
+  // optimizations.
+  // |context| may be NULL. If |context| is NULL, then the bitmap backing store
+  // is not initialized.
   static BitmapPlatformDevice* Create(CGContextRef context,
                                       int width, int height,
                                       bool is_opaque);
 
+  // Creates a BitmapPlatformDevice instance.  If |is_opaque| is false,
+  // then the bitmap is initialzed to 0.
+  static BitmapPlatformDevice* CreateAndClear(int width, int height,
+                                              bool is_opaque);
+
   // Creates a context for |data| and calls Create.
+  // If |data| is NULL, then the bitmap backing store is not initialized.
   static BitmapPlatformDevice* CreateWithData(uint8_t* data,
                                               int width, int height,
                                               bool is_opaque);