Add kGCCallbackFlagCollectAllExternalMemory for external memory limit triggered gc

BUG=chromium:570268,chromium:621829

Review-Url: https://codereview.chromium.org/2233683002
Cr-Commit-Position: refs/heads/master@{#38808}

include/v8.h

@@ -5246,6 +5246,7 @@ enum GCCallbackFlags {
   kGCCallbackFlagForced = 1 << 2,
   kGCCallbackFlagSynchronousPhantomCallbackProcessing = 1 << 3,
   kGCCallbackFlagCollectAllAvailableGarbage = 1 << 4,
+  kGCCallbackFlagCollectAllExternalMemory = 1 << 5,
 };
 
 typedef void (*GCCallback)(GCType type, GCCallbackFlags flags);

src/heap/heap.cc

@@ -907,7 +907,10 @@ void Heap::ReportExternalMemoryPressure(const char* gc_reason) {
     if (incremental_marking()->CanBeActivated()) {
       StartIncrementalMarking(
           i::Heap::kNoGCFlags,
-          kGCCallbackFlagSynchronousPhantomCallbackProcessing, gc_reason);
+          static_cast<GCCallbackFlags>(
+              kGCCallbackFlagSynchronousPhantomCallbackProcessing |
+              kGCCallbackFlagCollectAllExternalMemory),
+          gc_reason);
     } else {
       CollectAllGarbage(i::Heap::kNoGCFlags, gc_reason,
                         kGCCallbackFlagSynchronousPhantomCallbackProcessing);