[SAB] Move creation of SharedArrayBuffer/Atomics to InitializeGlobal

It is only attached to the global object if the --harmony-sharedarraybuffer
flag is enabled, but this allows more objects to be added to the snapshot which
seems to reduce the amount of heap memory used per context.

Bug: chromium:724053
Change-Id: I5d1115a0e3ed9abf41cb3ab80d19d622cbef7b93
Reviewed-on: https://chromium-review.googlesource.com/534594Reviewed-by: Adam Klein <adamk@chromium.org>
Commit-Queue: Ben Smith <binji@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45930}

src/contexts.h

@@ -219,6 +219,7 @@ enum ContextLookupFlags {
     async_generator_await_reject_shared_fun)                                   \
   V(ASYNC_GENERATOR_AWAIT_RESOLVE_SHARED_FUN, SharedFunctionInfo,              \
     async_generator_await_resolve_shared_fun)                                  \
+  V(ATOMICS_OBJECT, JSObject, atomics_object)                                  \
   V(BOOLEAN_FUNCTION_INDEX, JSFunction, boolean_function)                      \
   V(BOUND_FUNCTION_WITH_CONSTRUCTOR_MAP_INDEX, Map,                            \
     bound_function_with_constructor_map)                                       \