Add sk_calloc. Remove SkMemory_stdlib, which seems unused.

src/ports/SkMemory_mozalloc.cpp

Index: src/ports/SkMemory_mozalloc.cpp
diff --git a/src/ports/SkMemory_mozalloc.cpp b/src/ports/SkMemory_mozalloc.cpp
index 2c049b2a03039b9d956cad82fa75814d352efb58..4ca931267152706632740696b5b9092b877820d3 100644
--- a/src/ports/SkMemory_mozalloc.cpp
+++ b/src/ports/SkMemory_mozalloc.cpp
@@ -37,3 +37,11 @@ void sk_free(void* 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);
+}