Illumos doesn't have malloc_usable_size; just return the initial size.

--- webkitgtk-2.48.1/Source/ThirdParty/skia/src/ports/SkMemory_malloc.cpp.orig
+++ webkitgtk-2.48.1/Source/ThirdParty/skia/src/ports/SkMemory_malloc.cpp
@@ -127,7 +127,6 @@
         completeSize = malloc_usable_size(addr);
         SkASSERT(completeSize >= size);
     #elif defined(SK_BUILD_FOR_UNIX)
-        completeSize = malloc_usable_size(addr);
         SkASSERT(completeSize >= size);
     #elif defined(SK_BUILD_FOR_WIN)
         completeSize = _msize(addr);