Fix OOM handling on a background thread.

We don't have an Isolate, so we cannot use it for retrieving data or
calling the embedder's OOM handler. So just crash.

BUG=5525

Review-Url: https://codereview.chromium.org/2427623002
Cr-Commit-Position: refs/heads/master@{#40363}

src/api.cc

@@ -274,10 +274,23 @@ void i::V8::FatalProcessOutOfMemory(const char* location, bool is_heap_oom) {
   i::Isolate* isolate = i::Isolate::Current();
   char last_few_messages[Heap::kTraceRingBufferSize + 1];
   char js_stacktrace[Heap::kStacktraceBufferSize + 1];
+  i::HeapStats heap_stats;
+
+  if (isolate == nullptr) {
+    // On a background thread -> we cannot retrieve memory information from the
+    // Isolate. Write easy-to-recognize values on the stack.
+    memset(last_few_messages, 0x0badc0de, Heap::kTraceRingBufferSize + 1);
+    memset(js_stacktrace, 0x0badc0de, Heap::kStacktraceBufferSize + 1);
+    memset(&heap_stats, 0xbadc0de, sizeof(heap_stats));
+    // Note that the embedder's oom handler won't be called in this case. We
+    // just crash.
+    FATAL("API fatal error handler returned after process out of memory");
+    return;
+  }
+
   memset(last_few_messages, 0, Heap::kTraceRingBufferSize + 1);
   memset(js_stacktrace, 0, Heap::kStacktraceBufferSize + 1);
 
-  i::HeapStats heap_stats;
   intptr_t start_marker;
   heap_stats.start_marker = &start_marker;
   size_t new_space_size;