Remove deprecated web_task_runner.h.

It's unused as of now.

Bug: 826203
Change-Id: I8e54a8234cb83c1ac871768a62fb2224be9ba0d0
Reviewed-on: https://chromium-review.googlesource.com/c/1356461Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Alexander Timin <altimin@chromium.org>
Commit-Queue: Yuta Kitamura <yutak@chromium.org>
Cr-Commit-Position: refs/heads/master@{#612591}

third_party/blink/renderer/platform/BUILD.gn

@@ -1364,7 +1364,6 @@ jumbo_component("platform") {
     "web_mouse_event.cc",
     "web_mouse_wheel_event.cc",
     "web_pointer_event.cc",
-    "web_task_runner.h",
     "web_test_support.cc",
     "web_test_support.h",
     "web_text_input_info.cc",

third_party/blink/renderer/platform/web_task_runner.h

-// Copyright 2015 The Chromium Authors. All rights reserved.
-// Use of this source code is governed by a BSD-style license that can be
-// found in the LICENSE file.
-
-#ifndef THIRD_PARTY_BLINK_RENDERER_PLATFORM_WEB_TASK_RUNNER_H_
-#define THIRD_PARTY_BLINK_RENDERER_PLATFORM_WEB_TASK_RUNNER_H_
-
-#include "third_party/blink/renderer/platform/scheduler/public/post_cancellable_task.h"
-#include "third_party/blink/renderer/platform/scheduler/public/post_cross_thread_task.h"
-
-// The contents of this file are moved to the above headers. For future
-// includes, use either of them.
-
-#endif  // THIRD_PARTY_BLINK_RENDERER_PLATFORM_WEB_TASK_RUNNER_H_