Fix crash caused by NULL response_headers passed to StreamHandleImpl

This fixes a bug introduced in https://codereview.chromium.org/588643002
which caused a crash if NULL response_headers were passed to StreamhandleImpl.

Review URL: https://codereview.chromium.org/587173002

Cr-Commit-Position: refs/heads/master@{#296104}

content/browser/streams/stream_handle_impl.cc

@@ -30,11 +30,15 @@ StreamHandleImpl::StreamHandleImpl(
       url_(stream->url()),
       original_url_(original_url),
       mime_type_(mime_type),
-      // Make a copy of the response headers so it is safe to pass this across
-      // threads.
-      response_headers_(
-          new net::HttpResponseHeaders(response_headers->raw_headers())),
-      stream_message_loop_(base::MessageLoopProxy::current().get()) {}
+      response_headers_(NULL),
+      stream_message_loop_(base::MessageLoopProxy::current().get()) {
+  // Make a copy of the response headers so it is safe to pass this across
+  // threads.
+  if (response_headers.get()) {
+    response_headers_ =
+        new net::HttpResponseHeaders(response_headers->raw_headers());
+  }
+}
 
 StreamHandleImpl::~StreamHandleImpl() {
   stream_message_loop_->PostTaskAndReply(FROM_HERE,