Reland of Ship async functions (patchset #1 id:1 of https://codereview.chromium.org/2364963003/ )

Reason for revert:
Fixed underlying cause of TSAN issue; trying again while watching the bots.

Original issue's description:
> Revert of Ship async functions (patchset #2 id:20001 of https://codereview.chromium.org/2356943002/ )
>
> Reason for revert:
> Triggers TSAN errors on Linux64:
>
> https://build.chromium.org/p/client.v8/builders/V8%20Linux64%20TSAN/builds/11901
>
> Original issue's description:
> > Ship async functions
> >
> > Intent to ship discussion:
> > https://groups.google.com/forum/#!topic/v8-users/gwpcEIrgIZY
> >
> > BUG=v8:4483
> >
> > Committed: https://crrev.com/9f7540a0ec6333e7c71615609fe29274d9b7a4c5
> > Cr-Commit-Position: refs/heads/master@{#39697}
>
> TBR=caitp@igalia.com,littledan@chromium.org
> # Skipping CQ checks because original CL landed less than 1 days ago.
> NOPRESUBMIT=true
> NOTREECHECKS=true
> NOTRY=true
> BUG=v8:4483
>
> Committed: https://crrev.com/a4354b6c15fd9dbde2677efdf81fbc99ff0cf3e0
> Cr-Commit-Position: refs/heads/master@{#39699}

TBR=caitp@igalia.com,adamk@chromium.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=v8:4483

Review-Url: https://codereview.chromium.org/2363093003
Cr-Commit-Position: refs/heads/master@{#39702}

src/flag-definitions.h

@@ -213,7 +213,6 @@ DEFINE_IMPLICATION(es_staging, move_object_start)
 #define HARMONY_STAGED_BASE(V)                                               \
   V(harmony_regexp_lookbehind, "harmony regexp lookbehind")                  \
   V(harmony_tailcalls, "harmony tail calls")                                 \
-  V(harmony_async_await, "harmony async-await")                              \
   V(harmony_string_padding, "harmony String-padding methods")
 
 #ifdef V8_I18N_SUPPORT
@@ -227,6 +226,7 @@ DEFINE_IMPLICATION(es_staging, move_object_start)
 
 // Features that are shipping (turned on by default, but internal flag remains).
 #define HARMONY_SHIPPING(V)                                                  \
+  V(harmony_async_await, "harmony async-await")                              \
   V(harmony_restrictive_declarations,                                        \
     "harmony limitations on sloppy mode function declarations")              \
   V(harmony_object_values_entries, "harmony Object.values / Object.entries") \