Bug 885515 - Part 2: Add MOZ_HEAP_CLASS to mfbt, r=ehsan

mfbt/Attributes.h

@@ -397,10 +397,15 @@
  * MOZ_NONHEAP_CLASS: Applies to all classes. Any class with this annotation is
  *   expected to live on the stack or in static storage, so it is a compile-time
  *   error to use it, or an array of such objects, as the type of a new
- *   expression (unless placement new is being used). If a member of another
- *   class uses this class, or if another class inherits from this class, then
- *   it is considered to be a non-heap class as well, although this attribute
- *   need not be provided in such cases.
+ *   expression. If a member of another class uses this class, or if another
+ *   class inherits from this class, then it is considered to be a non-heap class
+ *   as well, although this attribute need not be provided in such cases.
+ * MOZ_HEAP_CLASS: Applies to all classes. Any class with this annotation is
+ *   expected to live on the heap, so it is a compile-time error to use it, or
+ *   an array of such objects, as the type of a variable declaration, or as a
+ *   temporary object. If a member of another class uses this class, or if
+ *   another class inherits from this class, then it is considered to be a heap
+ *   class as well, although this attribute need not be provided in such cases.
  * MOZ_ONLY_USED_TO_AVOID_STATIC_CONSTRUCTORS: Applies to all classes that are
  *   intended to prevent introducing static initializers.  This attribute
  *   currently makes it a compile-time error to instantiate these classes
@@ -475,6 +480,7 @@
 #  define MOZ_MUST_OVERRIDE __attribute__((annotate("moz_must_override")))
 #  define MOZ_STACK_CLASS __attribute__((annotate("moz_stack_class")))
 #  define MOZ_NONHEAP_CLASS __attribute__((annotate("moz_nonheap_class")))
+#  define MOZ_HEAP_CLASS __attribute__((annotate("moz_heap_class")))
 #  define MOZ_TRIVIAL_CTOR_DTOR __attribute__((annotate("moz_trivial_ctor_dtor")))
 #  ifdef DEBUG
      /* in debug builds, these classes do have non-trivial constructors. */
@@ -509,6 +515,7 @@
 #  define MOZ_MUST_OVERRIDE /* nothing */
 #  define MOZ_STACK_CLASS /* nothing */
 #  define MOZ_NONHEAP_CLASS /* nothing */
+#  define MOZ_HEAP_CLASS /* nothing */
 #  define MOZ_TRIVIAL_CTOR_DTOR /* nothing */
 #  define MOZ_ONLY_USED_TO_AVOID_STATIC_CONSTRUCTORS /* nothing */
 #  define MOZ_IMPLICIT /* nothing */