[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}

include/v8.h

@@ -10045,7 +10045,7 @@ 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) +
@@ -10063,7 +10063,11 @@ 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;