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

ui/gl/gl_implementation.cc

Index: ui/gl/gl_implementation.cc
diff --git a/ui/gl/gl_implementation.cc b/ui/gl/gl_implementation.cc
index 9235cb935e55ff4823b11ede7bbc9d399f3d3396..688d8b895e62c0cf79d9986bd1cd70f8a7eee6b5 100644
--- a/ui/gl/gl_implementation.cc
+++ b/ui/gl/gl_implementation.cc
@@ -14,6 +14,7 @@
 #include "base/logging.h"
 #include "base/macros.h"
 #include "base/memory/ptr_util.h"
+#include "base/strings/string_piece.h"
 #include "base/strings/string_split.h"
 #include "base/strings/string_util.h"
 #include "build/build_config.h"
@@ -162,10 +163,10 @@ std::string FilterGLExtensionList(
   if (extensions == NULL)
     return "";
 
-  std::vector<std::string> extension_vec = base::SplitString(
+  std::vector<base::StringPiece> extension_vec = base::SplitStringPiece(
       extensions, " ", base::TRIM_WHITESPACE, base::SPLIT_WANT_ALL);
 
-  auto is_disabled = [&disabled_extensions](const std::string& ext) {
+  auto is_disabled = [&disabled_extensions](const base::StringPiece& ext) {
     return std::find(disabled_extensions.begin(), disabled_extensions.end(),
                      ext) != disabled_extensions.end();
   };
@@ -201,7 +202,7 @@ std::string GetGLExtensionsFromCurrentContext(GLApi* api) {
   GLint num_extensions = 0;
   api->glGetIntegervFn(GL_NUM_EXTENSIONS, &num_extensions);
 
-  std::vector<std::string> exts(num_extensions);
+  std::vector<base::StringPiece> exts(num_extensions);
   for (GLint i = 0; i < num_extensions; ++i) {
     const char* extension =
         reinterpret_cast<const char*>(api->glGetStringiFn(GL_EXTENSIONS, i));