deps: V8: cherry-pick 5ebd6fcd from upstream

Original commit message:
  [heap] Lower external allocation limit when external memory shrinks.

  BUG=chromium:728228
  CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_chromium_rel_ng

  Review-Url: https://codereview.chromium.org/2921883002
  Cr-Commit-Position: refs/heads/master@{#45726}

PR-URL: #21269
Fixes: #21021
Reviewed-By: Myles Borins <myles.borins@gmail.com>

deps/v8/include/v8-version.h

@@ -11,7 +11,7 @@
 #define V8_MAJOR_VERSION 6
 #define V8_MINOR_VERSION 2
 #define V8_BUILD_NUMBER 414
-#define V8_PATCH_LEVEL 56
+#define V8_PATCH_LEVEL 57
 
 // Use 1 for candidates and 0 otherwise.
 // (Boolean macro values are not supported by all preprocessors.)

deps/v8/include/v8.h

@@ -10251,7 +10251,7 @@ int64_t Isolate::AdjustAmountOfExternalAllocatedMemory(
   typedef internal::Internals I;
   int64_t* external_memory = reinterpret_cast<int64_t*>(
       reinterpret_cast<uint8_t*>(this) + I::kExternalMemoryOffset);
-  const int64_t external_memory_limit = *reinterpret_cast<int64_t*>(
+  int64_t* external_memory_limit = reinterpret_cast<int64_t*>(
       reinterpret_cast<uint8_t*>(this) + I::kExternalMemoryLimitOffset);
   int64_t* external_memory_at_last_mc =
       reinterpret_cast<int64_t*>(reinterpret_cast<uint8_t*>(this) +
@@ -10269,7 +10269,11 @@ int64_t Isolate::AdjustAmountOfExternalAllocatedMemory(
     CheckMemoryPressure();
   }
 
-  if (change_in_bytes > 0 && amount > external_memory_limit) {
+  if (change_in_bytes < 0) {
+    *external_memory_limit += change_in_bytes;
+  }
+
+  if (change_in_bytes > 0 && amount > *external_memory_limit) {
     ReportExternalAllocationLimitReached();
   }
   return *external_memory;