Update the remaining include paths of base/string_piece.h to its new location.

ui/gfx/platform_font_pango.cc

 // Copyright (c) 2012 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.
 
 #include "ui/gfx/platform_font_pango.h"
 
 #include <fontconfig/fontconfig.h>
 #include <pango/pango.h>
 
 #include <algorithm>
 #include <string>
 
 #include "base/logging.h"
-#include "base/string_piece.h"
+#include "base/strings/string_piece.h"
 #include "base/strings/string_split.h"
 #include "base/utf_string_conversions.h"
 #include "grit/app_locale_settings.h"
 #include "third_party/skia/include/core/SkPaint.h"
 #include "third_party/skia/include/core/SkTypeface.h"
 #include "ui/base/l10n/l10n_util.h"
 #include "ui/gfx/canvas.h"
 #include "ui/gfx/font.h"
 #include "ui/gfx/pango_util.h"
 
@@ skipping 359 matching lines @@
   return new PlatformFontPango(native_font);
 }
 
 // static
 PlatformFont* PlatformFont::CreateFromNameAndSize(const std::string& font_name,
                                                   int font_size) {
   return new PlatformFontPango(font_name, font_size);
 }
 
 }  // namespace gfx