Fix a FileWriter abort race by turning an assert into a soft check.

webkit/fileapi/webfilewriter_base.cc

Index: webkit/fileapi/webfilewriter_base.cc
diff --git a/webkit/fileapi/webfilewriter_base.cc b/webkit/fileapi/webfilewriter_base.cc
index 7621a4fae8e5e42ad5e569ed56582b11677d43f1..26774038c60a22c3423690ec5529052d460a8745 100644
--- a/webkit/fileapi/webfilewriter_base.cc
+++ b/webkit/fileapi/webfilewriter_base.cc
@@ -54,7 +54,9 @@ void WebFileWriterBase::write(
 // thing to come back is the cancel response.  We only notify the
 // AsyncFileWriterClient when it's all over.
 void WebFileWriterBase::cancel() {
-  DCHECK(kOperationWrite == operation_ || kOperationTruncate == operation_);
+  // Check for the cancel passing the previous operation's return in-flight.
+  if (kOperationWrite != operation_ && kOperationTruncate != operation_)
+    return;
   if (kCancelNotInProgress != cancel_state_)
     return;
   cancel_state_ = kCancelSent;