base: 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://codereview.chromium.org/13374002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@191508 0039d316-1c4b-4281-b951-d872f2087c98
diff --git a/base/strings/utf_offset_string_conversions.cc b/base/strings/utf_offset_string_conversions.cc
index 5a6f0c0..bb402e4 100644
--- a/base/strings/utf_offset_string_conversions.cc
+++ b/base/strings/utf_offset_string_conversions.cc
@@ -7,7 +7,7 @@
 #include <algorithm>
 
 #include "base/memory/scoped_ptr.h"
-#include "base/string_piece.h"
+#include "base/strings/string_piece.h"
 #include "base/strings/utf_string_conversion_utils.h"
 
 namespace base {