[HTMLParser] Always start pumping new chunks after unpausing.

Pauses may happen while the parser has already yielded, and in
particular when a script-inserted style sheet appears that has
an external resource.

The failure mode before this patch was:
1. Yield & schedule unyield
2. Script task runs
3. Script task inserts stylesheet with external resource
4. Resource finished
5. ResumeParsingAfterPause called, but there is no last_chunk_before_pause_
because there was a yield, not a pause.

Bug:902237

Change-Id: Id65c721fc193ecec7190e6b466a6794b65ea6acc
Reviewed-on: https://chromium-review.googlesource.com/c/1389502Reviewed-by: Kouhei Ueno <kouhei@chromium.org>
Commit-Queue: Chris Harrelson <chrishtr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#622099}

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

@@ -1028,10 +1028,16 @@ void HTMLDocumentParser::ResumeParsingAfterPause() {
     return;
 
   if (have_background_parser_) {
+    // If we paused in the middle of processing a token chunk,
+    // deal with that before starting to pump.
     if (last_chunk_before_pause_) {
       ValidateSpeculations(std::move(last_chunk_before_pause_));
       DCHECK(!last_chunk_before_pause_);
       PumpPendingSpeculations();
+    } else if (!IsScheduledForUnpause()) {
+      // Otherwise, start pumping if we're not already scheduled to unpause
+      // already.
+      PumpPendingSpeculations();
     }
     return;
   }