Avoid WTF::Vector::at() and operator[] in core/html.

third_party/WebKit/Source/core/html/track/vtt/VTTRegionList.cpp

Index: third_party/WebKit/Source/core/html/track/vtt/VTTRegionList.cpp
diff --git a/third_party/WebKit/Source/core/html/track/vtt/VTTRegionList.cpp b/third_party/WebKit/Source/core/html/track/vtt/VTTRegionList.cpp
index e6f9904f6b7af94deeb73bd6ccfe1a25bfc34e21..d0671f07144b9ce7b7661530322780835e897b64 100644
--- a/third_party/WebKit/Source/core/html/track/vtt/VTTRegionList.cpp
+++ b/third_party/WebKit/Source/core/html/track/vtt/VTTRegionList.cpp
@@ -44,9 +44,9 @@ VTTRegion* VTTRegionList::getRegionById(const String& id) const {
   if (id.isEmpty())
     return nullptr;
 
-  for (size_t i = 0; i < m_list.size(); ++i) {
-    if (m_list[i]->id() == id)
-      return m_list[i].get();
+  for (const auto& region : m_list) {
+    if (region->id() == id)
+      return region.get();
   }
 
   return nullptr;