Change uses of base::JoinString to pass StringPieces where possible.

components/nacl/renderer/platform_info.cc

Index: components/nacl/renderer/platform_info.cc
diff --git a/components/nacl/renderer/platform_info.cc b/components/nacl/renderer/platform_info.cc
index d4eba29c92f072eeefece4a547c0fb039dbdea6e..24111cede2c3214e25cee789487df45ce334573f 100644
--- a/components/nacl/renderer/platform_info.cc
+++ b/components/nacl/renderer/platform_info.cc
@@ -4,6 +4,7 @@
 
 #include "base/cpu.h"
 #include "base/logging.h"
+#include "base/strings/string_piece.h"
 #include "base/strings/string_util.h"
 #include "build/build_config.h"
 #if defined(OS_WIN)
@@ -59,7 +60,7 @@ std::string GetCpuFeatures() {
   //           doesn't handle other architectures very well, and we
   //           should at least detect the presence of ARM's integer
   //           divide.
-  std::vector<std::string> features;
+  std::vector<base::StringPiece> features;
   base::CPU cpu;
 
   // On x86, SSE features are ordered: the most recent one implies the