update_engine: Use C++11 range-based for loop when possible.

This cleanup just simplifies the notation of several for loops using
the new range-based syntax.

BUG=None
TEST=`FEATURES=test USE="lang clang" emerge-link update_engine`

Change-Id: I2120a1b1c3fd00e9a02a62abfa8427932080d0f9
Reviewed-on: https://chromium-review.googlesource.com/227897
Tested-by: Alex Deymo <deymo@chromium.org>
Reviewed-by: Alex Vakulenko <avakulenko@chromium.org>
Commit-Queue: Alex Deymo <deymo@chromium.org>
diff --git a/chrome_browser_proxy_resolver.cc b/chrome_browser_proxy_resolver.cc
index cd0e2a7..7696d4b 100644
--- a/chrome_browser_proxy_resolver.cc
+++ b/chrome_browser_proxy_resolver.cc
@@ -104,10 +104,9 @@
   }
 
   // Kill outstanding timers
-  for (TimeoutsMap::iterator it = timers_.begin(), e = timers_.end(); it != e;
-       ++it) {
-    g_source_destroy(it->second);
-    it->second = nullptr;
+  for (auto& timer : timers_) {
+    g_source_destroy(timer.second);
+    timer.second = nullptr;
   }
 }