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

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

Index: third_party/WebKit/Source/core/html/track/TextTrackContainer.cpp
diff --git a/third_party/WebKit/Source/core/html/track/TextTrackContainer.cpp b/third_party/WebKit/Source/core/html/track/TextTrackContainer.cpp
index 93053b6d5f7846ec9f73b28e570c6b4704030dd0..f5bf1fcc1dbac936fff2b804114640bf8c961ae0 100644
--- a/third_party/WebKit/Source/core/html/track/TextTrackContainer.cpp
+++ b/third_party/WebKit/Source/core/html/track/TextTrackContainer.cpp
@@ -109,8 +109,8 @@ void TextTrackContainer::updateDisplay(HTMLMediaElement& mediaElement,
   // corresponding CSS boxes added to output, in text track cue order, run the
   // following substeps:
   double movieTime = video.currentTime();
-  for (size_t i = 0; i < activeCues.size(); ++i) {
-    TextTrackCue* cue = activeCues[i].data();
+  for (const auto& activeCue : activeCues) {
+    TextTrackCue* cue = activeCue.data();
 
     DCHECK(cue->isActive());
     if (!cue->track() || !cue->track()->isRendered() || !cue->isActive())