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

string_piece.h was moved into base/strings/ in r191206 -
https://chromiumcodereview.appspot.com/12982018/

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Review URL: https://chromiumcodereview.appspot.com/13892006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@193845 0039d316-1c4b-4281-b951-d872f2087c98
diff --git a/content/browser/host_zoom_map_impl.cc b/content/browser/host_zoom_map_impl.cc
index a60a923..ad70977 100644
--- a/content/browser/host_zoom_map_impl.cc
+++ b/content/browser/host_zoom_map_impl.cc
@@ -2,11 +2,11 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-#include <cmath>
-
 #include "content/browser/host_zoom_map_impl.h"
 
-#include "base/string_piece.h"
+#include <cmath>
+
+#include "base/strings/string_piece.h"
 #include "base/utf_string_conversions.h"
 #include "base/values.h"
 #include "content/browser/renderer_host/render_process_host_impl.h"