tools/gn: Convert for loops to use the new range-based loops in C++11.

tools/gn/command_clean.cc

Index: tools/gn/command_clean.cc
diff --git a/tools/gn/command_clean.cc b/tools/gn/command_clean.cc
index 8872478f120f3d89637dff639d0fb3775fbf67f8..cef3526288ec03cd1df87b076bac67bf30da9a16 100644
--- a/tools/gn/command_clean.cc
+++ b/tools/gn/command_clean.cc
@@ -30,10 +30,10 @@ std::string ExtractGNBuildCommands(const base::FilePath& build_ninja_file) {
 
   std::string result;
   int num_blank_lines = 0;
-  for (size_t i = 0; i < lines.size(); ++i) {
-    result += lines[i];
+  for (const auto& line : lines) {
+    result += line;
     result += "\n";
-    if (lines[i].empty()) {
+    if (line.empty()) {
       ++num_blank_lines;
     }
     if (num_blank_lines == 2)