Add sk_calloc. Remove SkMemory_stdlib, which seems unused.

src/ports/SkMemory_mozalloc.cpp

 /*
  * Copyright 2011 Google Inc.
  * Copyright 2012 Mozilla Foundation
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkTypes.h"
 
@@ skipping 19 matching lines @@
     return moz_xrealloc(addr, size);
 }
 
 void sk_free(void* p) {
     moz_free(p);
 }
 
 void* sk_malloc_flags(size_t size, unsigned flags) {
     return (flags & SK_MALLOC_THROW) ? moz_xmalloc(size) : moz_malloc(size);
 }
+
+void* sk_calloc(size_t size) {
+    return moz_calloc(size, 1);
+}
+
+void* sk_calloc_throw(size_t size) {
+    return moz_xcalloc(size, 1);
+}