Optionally clear PlatformCanvas instances.

skia/ext/bitmap_platform_device_mac.cc

Index: skia/ext/bitmap_platform_device_mac.cc
===================================================================
--- skia/ext/bitmap_platform_device_mac.cc	(revision 126450)
+++ skia/ext/bitmap_platform_device_mac.cc	(working copy)
@@ -125,11 +125,6 @@
     bitmap.setPixels(data);
   } else {
     data = bitmap.getPixels();
-
-    // Note: The Windows implementation clears the Bitmap later on.
-    // This bears mentioning since removal of this line makes the
-    // unit tests only fail periodically (or when MallocPreScribble is set).
-    bitmap.eraseARGB(0, 0, 0, 0);
   }
 
   bitmap.setIsOpaque(is_opaque);
@@ -242,7 +237,10 @@
     SkBitmap::Config config, int width, int height, bool isOpaque,
     Usage /*usage*/) {
   SkASSERT(config == SkBitmap::kARGB_8888_Config);
-  return BitmapPlatformDevice::Create(NULL, width, height, isOpaque);
+  SkDevice* bitmap_device = BitmapPlatformDevice::Create(NULL, width, height,
+                                                         isOpaque);
+  bitmap_device->accessBitmap(true).eraseARGB(0, 0, 0, 0);
+  return bitmap_device;
 }
 
 }  // namespace skia