Reland "Ship globalThis 🎉"

This is a reland of 4dac9872

Original change's description:
> Ship globalThis 🎉
>
> Proposal repository:
> https://github.com/tc39/proposal-global
>
> Intent to ship:
> https://groups.google.com/d/msg/v8-users/Vkoh0wXRwaM/Yt7MpzhkAgAJ
>
> Bug: v8:5537
> Change-Id: I60a6c5375165d89548db12fef454a64137d04c27
> Reviewed-on: https://chromium-review.googlesource.com/1195494
> Reviewed-by: Adam Klein <adamk@chromium.org>
> Reviewed-by: Sathya Gunasekaran <gsathya@chromium.org>
> Commit-Queue: Mathias Bynens <mathias@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#55543}

TBR=adamk@chromium.org,machenbach@chromium.org,gsathya@chromium.org,mathias@chromium.org

No-Presubmit: true
Bug: v8:5537
Change-Id: I9776c24952e472c59861ee07f78b6456b19e6f2a
Reviewed-on: https://chromium-review.googlesource.com/1203894
Commit-Queue: Mathias Bynens <mathias@chromium.org>
Reviewed-by: Mathias Bynens <mathias@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55635}

src/flag-definitions.h

@@ -230,8 +230,7 @@ DEFINE_IMPLICATION(harmony_class_fields, harmony_private_fields)
   V(harmony_public_fields, "harmony public fields in class literals")      \
   V(harmony_private_fields, "harmony private fields in class literals")    \
   V(harmony_numeric_separator, "harmony numeric separator between digits") \
-  V(harmony_string_matchall, "harmony String.prototype.matchAll")          \
-  V(harmony_global, "harmony global")
+  V(harmony_string_matchall, "harmony String.prototype.matchAll")
 
 // Features that are shipping (turned on by default, but internal flag remains).
 #define HARMONY_SHIPPING(V)                                              \
@@ -243,7 +242,8 @@ DEFINE_IMPLICATION(harmony_class_fields, harmony_private_fields)
   V(harmony_dynamic_import, "harmony dynamic import")                    \
   V(harmony_array_prototype_values, "harmony Array.prototype.values")    \
   V(harmony_array_flat, "harmony Array.prototype.{flat,flatMap}")        \
-  V(harmony_symbol_description, "harmony Symbol.prototype.description")
+  V(harmony_symbol_description, "harmony Symbol.prototype.description")  \
+  V(harmony_global, "harmony globalThis")
 
 // Once a shipping feature has proved stable in the wild, it will be dropped
 // from HARMONY_SHIPPING, all occurrences of the FLAG_ variable are removed,

test/mjsunit/harmony/global.js

@@ -13,7 +13,7 @@ assertEquals(globalThis.globalThis.globalThis.globalThis, this);
 {
   const realm = Realm.create();
   assertEquals(Realm.global(realm), Realm.eval(realm, 'globalThis'));
-  assertTrue(Realm.global(realm) !== globalThis);
+  assertNotEquals(Realm.global(realm), globalThis);
 }
 
 {