Remove all occurrences of "using std::string".

webrtc/tools/frame_editing/frame_editing_lib.cc

Index: webrtc/tools/frame_editing/frame_editing_lib.cc
diff --git a/webrtc/tools/frame_editing/frame_editing_lib.cc b/webrtc/tools/frame_editing/frame_editing_lib.cc
index bb6a75edaab30e01850107de78a7ab6ea28bc8f1..f68dc1ffe9afa9a77d5b7498562fc135380cb0dc 100644
--- a/webrtc/tools/frame_editing/frame_editing_lib.cc
+++ b/webrtc/tools/frame_editing/frame_editing_lib.cc
@@ -18,13 +18,11 @@
 #include "webrtc/tools/frame_editing/frame_editing_lib.h"
 #include "webrtc/typedefs.h"
 
-using std::string;
-
 namespace webrtc {
 
-int EditFrames(const string& in_path, int width, int height,
+int EditFrames(const std::string& in_path, int width, int height,
                int first_frame_to_process, int interval,
-               int last_frame_to_process, const string& out_path) {
+               int last_frame_to_process, const std::string& out_path) {
   if (last_frame_to_process < first_frame_to_process) {
     fprintf(stderr, "The set of frames to cut is empty! (l < f)\n");
     return -10;