Check the weakptr parser before calling into it

This code used to bind a callback which did this implicity.

Bug: 845104
Change-Id: I75db3232daaf6d20b5caa01361913663bf582d56
Reviewed-on: https://chromium-review.googlesource.com/1067358Reviewed-by: Kouhei Ueno <kouhei@chromium.org>
Commit-Queue: Charlie Harrison <csharrison@chromium.org>
Cr-Commit-Position: refs/heads/master@{#560922}

third_party/blink/renderer/core/html/parser/background_html_parser.cc

@@ -137,7 +137,8 @@ void BackgroundHTMLParser::UpdateDocument(const String& decoded_data) {
     last_seen_encoding_data_ = encoding_data;
 
     xss_auditor_->SetEncoding(encoding_data.Encoding());
-    parser_->DidReceiveEncodingDataFromBackgroundParser(encoding_data);
+    if (parser_)
+      parser_->DidReceiveEncodingDataFromBackgroundParser(encoding_data);
   }
 
   if (decoded_data.IsEmpty())