Move more uses of string16 to specify base::

This makes base, ui, and net compile without the "using base::string16" directive that's currently checked in.

BUG=

Review URL: https://codereview.chromium.org/100303003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@238246 0039d316-1c4b-4281-b951-d872f2087c98
diff --git a/base/pickle_unittest.cc b/base/pickle_unittest.cc
index 6e9d41d..b1c5925 100644
--- a/base/pickle_unittest.cc
+++ b/base/pickle_unittest.cc
@@ -10,6 +10,9 @@
 #include "base/strings/string16.h"
 #include "testing/gtest/include/gtest/gtest.h"
 
+// Remove when this file is in the base namespace.
+using base::string16;
+
 namespace {
 
 const int testint = 2093847192;