Use a per-frame task runner at PrintRenderFrameHelper

This CL switches task runners at several places from per-thread to
per-frame. The purpose of this change is to make it possible to freeze
tasks for bfcache.

We found an actual case that JavaScript is executed:
browser_tests --gtest_filter=PrintBrowserTest.SelectionContainsIframe

Bug: 870606
Change-Id: If7027d9a104513408f2d14507f6938308273bd7b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1529501Reviewed-by: Alexander Timin <altimin@chromium.org>
Reviewed-by: Wei Li <weili@chromium.org>
Commit-Queue: Hajime Hoshi <hajimehoshi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#642359}

components/printing/renderer/print_render_frame_helper.cc

@@ -945,8 +945,10 @@ void PrepareFrameAndViewForPrint::DidStopLoading() {
   DCHECK(!on_ready_.is_null());
   // Don't call callback here, because it can delete |this| and WebView that is
   // called didStopLoading.
-  base::ThreadTaskRunnerHandle::Get()->PostTask(
-      FROM_HERE, base::BindOnce(&PrepareFrameAndViewForPrint::CallOnReady,
+  frame()
+      ->GetTaskRunner(blink::TaskType::kInternalDefault)
+      ->PostTask(FROM_HERE,
+                 base::BindOnce(&PrepareFrameAndViewForPrint::CallOnReady,
                                 weak_ptr_factory_.GetWeakPtr()));
 }