Introduce CSSLayoutSetMemoryFuncs

Summary:
- Add CSSLayoutSetMemoryFuncs function which allows application to replace malloc, calloc, realloc and free with application's functions, like zalloc and zfree of zlib.
- Fixed memory leaks in tests.
- Close #247

For example, to use dlmalloc with USE_DL_PREFIX

    CSSLayoutSetMemoryFuncs(&dlmalloc, &dlcalloc, &dlrealloc, &dlfree);

Reviewed By: emilsjolander

Differential Revision: D4178386

fbshipit-source-id: a79dbdaf82a512f42cc43f99dbc49faba296903b

tests/CSSLayoutDirtyMarkingTest.cpp

@@ -83,11 +83,14 @@ TEST(CSSLayoutTest, dirty_node_only_if_children_are_actually_removed) {
 
   CSSNodeCalculateLayout(root, CSSUndefined, CSSUndefined, CSSDirectionLTR);
 
-  CSSNodeRemoveChild(root, CSSNodeNew());
+  const CSSNodeRef child1 = CSSNodeNew();
+  CSSNodeRemoveChild(root, child1);
   EXPECT_FALSE(CSSNodeIsDirty(root));
+  CSSNodeFree(child1);
 
   CSSNodeRemoveChild(root, child0);
   EXPECT_TRUE(CSSNodeIsDirty(root));
+  CSSNodeFree(child0);
 
   CSSNodeFreeRecursive(root);
 }