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

third_party/WebKit/Source/core/html/track/LoadableTextTrack.cpp

Index: third_party/WebKit/Source/core/html/track/LoadableTextTrack.cpp
diff --git a/third_party/WebKit/Source/core/html/track/LoadableTextTrack.cpp b/third_party/WebKit/Source/core/html/track/LoadableTextTrack.cpp
index a9357ed80a4a5fcf7959ef238cb9bfae9dfc0570..6f7872b19fc9f0cc4c109bf350944bc70cec8e71 100644
--- a/third_party/WebKit/Source/core/html/track/LoadableTextTrack.cpp
+++ b/third_party/WebKit/Source/core/html/track/LoadableTextTrack.cpp
@@ -55,9 +55,9 @@ void LoadableTextTrack::setMode(const AtomicString& mode) {
 
 void LoadableTextTrack::addRegions(
     const HeapVector<Member<VTTRegion>>& newRegions) {
-  for (size_t i = 0; i < newRegions.size(); ++i) {
-    newRegions[i]->setTrack(this);
-    regions()->add(newRegions[i]);
+  for (const auto& region : newRegions) {
+    region->setTrack(this);
+    regions()->add(region);
   }
 }