deps: cherry-pick 2c75616 from upstream V8

Original commit message:

    [heap] Ensure progress in unmapping memory chunks.

    If sweeping is not making progress and there are many young generation
    GCs happening, then this can lead to accumulation of memory chunks in
    the unmapper queue.

    Bug: chromium:771966
    Change-Id: Ief73ada0d17198a80b668850c6d2e7ea413113e7
    Reviewed-on: https://chromium-review.googlesource.com/702479
    Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
    Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
    Cr-Commit-Position: refs/heads/master@{#48312}

Refs: v8/v8@2c75616
Refs: nodejs/help#917 (comment)
Refs: https://bugs.chromium.org/p/chromium/issues/detail?id=771966

PR-URL: #16490
Backport-PR-URL: #16569
Reviewed-By: Franziska Hinkelmann <franziska.hinkelmann@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Ali Ijaz Sheikh <ofrobots@google.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>

deps/v8/include/v8-version.h

@@ -11,7 +11,7 @@
 #define V8_MAJOR_VERSION 6
 #define V8_MINOR_VERSION 1
 #define V8_BUILD_NUMBER 534
-#define V8_PATCH_LEVEL 42
+#define V8_PATCH_LEVEL 43
 
 // Use 1 for candidates and 0 otherwise.
 // (Boolean macro values are not supported by all preprocessors.)

deps/v8/src/heap/heap.cc

@@ -1716,6 +1716,12 @@ void Heap::Scavenge() {
   IncrementalMarking::PauseBlackAllocationScope pause_black_allocation(
       incremental_marking());
 
+  if (mark_compact_collector()->sweeper().sweeping_in_progress() &&
+      memory_allocator_->unmapper()->NumberOfDelayedChunks() >
+          kMaxSemiSpaceSizeInKB / Page::kPageSize) {
+    mark_compact_collector()->EnsureSweepingCompleted();
+  }
+
   mark_compact_collector()->sweeper().EnsureNewSpaceCompleted();
 
   SetGCState(SCAVENGE);

deps/v8/src/heap/spaces.h

@@ -1226,6 +1226,11 @@ class V8_EXPORT_PRIVATE MemoryAllocator {
 
     bool has_delayed_chunks() { return delayed_regular_chunks_.size() > 0; }
 
+    int NumberOfDelayedChunks() {
+      base::LockGuard<base::Mutex> guard(&mutex_);
+      return static_cast<int>(delayed_regular_chunks_.size());
+    }
+
    private:
     static const int kReservedQueueingSlots = 64;
     static const int kMaxUnmapperTasks = 24;