Pass approprite task runners to AssociatedBinding::Bind via FindInPage

This is part of efforts to replace base::ThreadTaskRunnerHandle::Get()
and SequencedTaskRunnerHandle::Get() with other appropriate task runners
in the renderer.

Bug: 786332
Change-Id: Ib271cb86d37c55cc1ad50e24003c8df675976d08
Reviewed-on: https://chromium-review.googlesource.com/c/1293429Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Alexander Timin <altimin@chromium.org>
Commit-Queue: Hajime Hoshi <hajimehoshi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#601849}

third_party/blink/renderer/core/frame/find_in_page.cc

@@ -307,7 +307,8 @@ WebPlugin* FindInPage::GetWebPluginForFind() {
 
 void FindInPage::BindToRequest(
     mojom::blink::FindInPageAssociatedRequest request) {
-  binding_.Bind(std::move(request));
+  binding_.Bind(std::move(request),
+                frame_->GetTaskRunner(blink::TaskType::kInternalDefault));
 }
 
 void FindInPage::Dispose() {