writepixels needs to bump genID

TBR=

Author: reed@google.com

Review URL: https://codereview.chromium.org/352573005
diff --git a/src/core/SkCanvas.cpp b/src/core/SkCanvas.cpp
index ec98b7e..2fb5c95 100644
--- a/src/core/SkCanvas.cpp
+++ b/src/core/SkCanvas.cpp
@@ -753,6 +753,9 @@
     // here x,y are either 0 or negative
     pixels = ((const char*)pixels - y * rowBytes - x * info.bytesPerPixel());
 
+    // Tell our owning surface to bump its generation ID
+    this->predrawNotify();
+
     // The device can assert that the requested area is always contained in its bounds
     return device->writePixels(info, pixels, rowBytes, target.x(), target.y());
 }