Add additional checks for script streaming.

Add additional checks to script streaming, to help diagnose crbug.com/784326.

Bug: 784326
Change-Id: I19aacefc260dce0f5836fa14a217701d388f566c
Reviewed-on: https://chromium-review.googlesource.com/874191Reviewed-by: Kouhei Ueno <kouhei@chromium.org>
Commit-Queue: Daniel Vogelheim <vogelheim@chromium.org>
Cr-Commit-Position: refs/heads/master@{#530467}

third_party/WebKit/Source/core/script/ClassicPendingScript.cpp

@@ -148,6 +148,8 @@ void ClassicPendingScript::CancelStreaming() {
   streamer_->Cancel();
   streamer_ = nullptr;
   streamer_done_.Reset();
+  is_currently_streaming_ = false;
+  DCHECK(!IsCurrentlyStreaming());
 }
 
 void ClassicPendingScript::NotifyFinished(Resource* resource) {
@@ -324,6 +326,13 @@ void ClassicPendingScript::AdvanceReadyState(ReadyState new_ready_state) {
 
   // Streaming-related post conditions:
 
+  // To help diagnose crbug.com/78426, we'll temporarily add some DCHECKs
+  // that are a subset of the DCHECKs below:
+  if (IsCurrentlyStreaming()) {
+    DCHECK(streamer_);
+    DCHECK(!streamer_->IsFinished());
+  }
+
   // IsCurrentlyStreaming should match what streamer_ thinks.
   DCHECK_EQ(IsCurrentlyStreaming(), streamer_ && !streamer_->IsFinished());
   // IsCurrentlyStreaming should match the ready_state_.