Use 16 instead of 4 for determining member offsets.

This avoids UBSan warnings about unaligned memory access.

BUG=

Review URL: https://codereview.chromium.org/734873002

Cr-Commit-Position: refs/heads/master@{#25396}

src/base/macros.h

@@ -20,9 +20,9 @@
 // corresponds to 'offsetof' (in stddef.h), except that it doesn't
 // use 0 or NULL, which causes a problem with the compiler warnings
 // we have enabled (which is also why 'offsetof' doesn't seem to work).
-// Here we simply use the non-zero value 4, which seems to work.
-#define OFFSET_OF(type, field)                                          \
-  (reinterpret_cast<intptr_t>(&(reinterpret_cast<type*>(4)->field)) - 4)
+// Here we simply use the aligned, non-zero value 16.
+#define OFFSET_OF(type, field) \
+  (reinterpret_cast<intptr_t>(&(reinterpret_cast<type*>(16)->field)) - 16)
 
 
 #if V8_OS_NACL

src/heap/heap-inl.h

@@ -587,7 +587,7 @@ bool Heap::CollectGarbage(AllocationSpace space, const char* gc_reason,
 Isolate* Heap::isolate() {
   return reinterpret_cast<Isolate*>(
       reinterpret_cast<intptr_t>(this) -
-      reinterpret_cast<size_t>(reinterpret_cast<Isolate*>(4)->heap()) + 4);
+      reinterpret_cast<size_t>(reinterpret_cast<Isolate*>(16)->heap()) + 16);
 }