[clang-tidy] Apply modernize-use-auto in /chrome/browser/extensions

This change applies clang-tidy's modernize-use-auto [1] in /chrome/browser/extensions.
This change does not rewrite new and cast expressions.

Reproduction steps:
- run clang-tidy's modernize-use-auto
- run git cl format
- manually remove unused typedefs due to -Wunused-local-typedef error

[1] https://clang.llvm.org/extra/clang-tidy/checks/modernize-use-auto.html

This CL was uploaded by git cl split.

[email protected]

Bug: 890902
Change-Id: Ic5efd3b6eefa2c950dcb85793e824835e6f79c32
Reviewed-on: https://chromium-review.googlesource.com/c/1257903
Reviewed-by: Devlin <[email protected]>
Commit-Queue: Jan Wilken Dörrie <[email protected]>
Cr-Commit-Position: refs/heads/master@{#595988}
diff --git a/chrome/browser/extensions/default_apps.cc b/chrome/browser/extensions/default_apps.cc
index aafdde0..0c3a25a 100644
--- a/chrome/browser/extensions/default_apps.cc
+++ b/chrome/browser/extensions/default_apps.cc
@@ -156,8 +156,8 @@
         new_default_apps.insert(i.key());
     }
     // Filter out the new default apps for migrating users.
-    for (std::set<std::string>::iterator it = new_default_apps.begin();
-         it != new_default_apps.end(); ++it) {
+    for (auto it = new_default_apps.begin(); it != new_default_apps.end();
+         ++it) {
       prefs->Remove(*it, NULL);
     }
   }