Fix crash in WritableStreamFinishClose

third_party/WebKit/Source/core/streams/WritableStream.js

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // Implementation of WritableStream for Blink.  See
 // https://streams.spec.whatwg.org/#ws. The implementation closely follows the
 // standard, except where required for performance or integration with Blink. In
 // particular, classes, methods and abstract operations are implemented in the
 // same order as in the standard, to simplify side-by-side reading.
 
@@ skipping 288 matching lines @@
       }
       v8.markPromiseAsHandled(writer[_readyPromise]);
     }
   }
 
   function WritableStreamFinishClose(stream) {
     const state = stream[_state];
     TEMP_ASSERT(state === CLOSING || state === ERRORED,
                 'state is "closing" or "errored"');
 
+    const writer = stream[_writer];
     if (state === CLOSING) {
-      v8.resolvePromise(stream[_writer][_closedPromise], undefined);
+      if (writer !== undefined) {
+        v8.resolvePromise(writer[_closedPromise], undefined);
+      }
       stream[_state] = CLOSED;
-    } else {
+    } else if (writer !== undefined) {
       TEMP_ASSERT(state === ERRORED, 'state is "errored"');
-      v8.rejectPromise(stream[_writer][_closedPromise], stream[_storedError]);
-      v8.markPromiseAsHandled(stream[_writer][_closedPromise]);
+      v8.rejectPromise(writer[_closedPromise], stream[_storedError]);
+      v8.markPromiseAsHandled(writer[_closedPromise]);
     }
 
     if (stream[_pendingAbortRequest] !== undefined) {
       v8.resolvePromise(stream[_pendingAbortRequest], undefined);
       stream[_pendingAbortRequest] = undefined;
     }
   }
 
   function WritableStreamRejectPromisesInReactionToError(stream) {
     TEMP_ASSERT(stream[_state] === ERRORED, 'stream.[[state]] is "errored"');
@@ skipping 599 matching lines @@
 
   defineProperty(global, 'WritableStream', {
     value: WritableStream,
     enumerable: false,
     configurable: true,
     writable: true
   });
 
   // TODO(ricea): Exports to Blink
 });