fixup: ensure HTMLPreloadScanner construction on Append

Attempts to fix an ASAN-detected crash for IsPrefetchOnly() Documents.

Bug: 901056, 1164226, 1160665, 1164377
Change-Id: I74c75c58d3e2d9319f67997e3b08d1ab7d5e008d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2618362Reviewed-by: Mason Freed <masonfreed@chromium.org>
Commit-Queue: Richard Townsend <richard.townsend@arm.com>
Cr-Commit-Position: refs/heads/master@{#841588}

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

@@ -1229,7 +1229,7 @@ void HTMLDocumentParser::Append(const String& input_source) {
 
   const SegmentedString source(input_source);
 
-  if (!preload_scanner_ && preloader_ && GetDocument()->Url().IsValid() &&
+  if (!preload_scanner_ && GetDocument()->Url().IsValid() &&
       (!task_runner_state_->IsSynchronous() ||
        GetDocument()->IsPrefetchOnly() || IsPaused())) {
     // If we're operating with synchronous, budgeted foreground HTML parsing