v8: backport 4f82f1d948c from upstream v8

Original commit message:
    [Api] Add an idle time garbage collection callback flag to GCCallbackFlags.

    BUG=chromium:718484

    Review-Url: https://codereview.chromium.org/2867073002
    Cr-Commit-Position: refs/heads/master@{#45167}

PR-URL: #13217
Reviewed-By: Franziska Hinkelmann <franziska.hinkelmann@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>

deps/v8/include/v8.h

@@ -6180,6 +6180,8 @@ enum GCType {
  *   - kGCCallbackFlagCollectAllAvailableGarbage: The GC callback is called
  *     in a phase where V8 is trying to collect all available garbage
  *     (e.g., handling a low memory notification).
+ *   - kGCCallbackScheduleIdleCollectGarbage: The GC callback is called to
+ *     trigger an idle garbage collection.
  */
 enum GCCallbackFlags {
   kNoGCCallbackFlags = 0,
@@ -6188,6 +6190,7 @@ enum GCCallbackFlags {
   kGCCallbackFlagSynchronousPhantomCallbackProcessing = 1 << 3,
   kGCCallbackFlagCollectAllAvailableGarbage = 1 << 4,
   kGCCallbackFlagCollectAllExternalMemory = 1 << 5,
+  kGCCallbackScheduleIdleCollectGarbage = 1 << 6,
 };
 
 typedef void (*GCCallback)(GCType type, GCCallbackFlags flags);