Use a direct include of strings headers in base/.

BUG=247723
TEST=none
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Review URL: https://chromiumcodereview.appspot.com/16320009

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@205284 0039d316-1c4b-4281-b951-d872f2087c98
diff --git a/base/strings/string_piece_unittest.cc b/base/strings/string_piece_unittest.cc
index 4a5548f..84ed9ef 100644
--- a/base/strings/string_piece_unittest.cc
+++ b/base/strings/string_piece_unittest.cc
@@ -4,7 +4,7 @@
 
 #include <string>
 
-#include "base/string16.h"
+#include "base/strings/string16.h"
 #include "base/strings/string_piece.h"
 #include "base/strings/utf_string_conversions.h"
 #include "testing/gtest/include/gtest/gtest.h"