Convert NULL and 0 to nullptr.

This was created by looking at warnings produced by clang's
-Wzero-as-null-pointer-constant. This updates most issues in
Skia code. However, there are places where GL and Vulkan want
pointer values which are explicitly 0, external headers which
use NULL directly, and possibly more uses in un-compiled
sources (for other platforms).

Change-Id: Id22fbac04d5c53497a53d734f0896b4f06fe8345
Reviewed-on: https://skia-review.googlesource.com/39521
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Ben Wagner <bungeman@google.com>
diff --git a/include/gpu/GrGpuResource.h b/include/gpu/GrGpuResource.h
index 677460e..6d28c12 100644
--- a/include/gpu/GrGpuResource.h
+++ b/include/gpu/GrGpuResource.h
@@ -154,7 +154,7 @@
      * @return true if the object has been released or abandoned,
      *         false otherwise.
      */
-    bool wasDestroyed() const { return NULL == fGpu; }
+    bool wasDestroyed() const { return nullptr == fGpu; }
 
     /**
      * Retrieves the context that owns the object. Note that it is possible for