webkit: Cleanup: Delete webkit_common_export.h

After recent refactorings moving almost the remaining pieces into
storage/, looks like nobody is including this header.

So we can remove it now.

BUG=338338
TEST=None
TBR=jamesr@chromium.org

Review URL: https://codereview.chromium.org/555323003

Cr-Commit-Position: refs/heads/master@{#294333}

webkit/common/webkit_common_export.h

-// Copyright (c) 2013 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 WEBKIT_COMMON_WEBKIT_COMMON_EXPORT_H_
-#define WEBKIT_COMMON_WEBKIT_COMMON_EXPORT_H_
-
-#if defined(COMPONENT_BUILD)
-#if defined(WIN32)
-
-#if defined(WEBKIT_COMMON_IMPLEMENTATION)
-#define WEBKIT_COMMON_EXPORT __declspec(dllexport)
-#else
-#define WEBKIT_COMMON_EXPORT __declspec(dllimport)
-#endif  // defined(WEBKIT_COMMON_IMPLEMENTATION)
-
-#else // defined(WIN32)
-#if defined(WEBKIT_COMMON_IMPLEMENTATION)
-#define WEBKIT_COMMON_EXPORT __attribute__((visibility("default")))
-#else
-#define WEBKIT_COMMON_EXPORT
-#endif
-#endif
-
-#else // defined(COMPONENT_BUILD)
-#define WEBKIT_COMMON_EXPORT
-#endif
-
-#endif  // WEBKIT_COMMON_WEBKIT_COMMON_EXPORT_H_