BackForwardCache: Refactoring: Use back() instead of an index

Bug: 1014174
Change-Id: I27fc6bcfb0a0bfa2937f9ad26a1bdc2e984dd803
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2251858
Commit-Queue: Hajime Hoshi <hajimehoshi@chromium.org>
Reviewed-by: Nicolás Peña Moreno <npm@chromium.org>
Reviewed-by: Alexander Timin <altimin@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#780242}

third_party/blink/renderer/core/paint/paint_timing.cc

@@ -289,11 +289,9 @@ void PaintTiming::SetFirstPaintAfterBackForwardCacheRestoreSwap(
     base::TimeTicks stamp) {
   // The last element is already allocated when the page is restored from the
   // cache.
-  size_t last_index =
-      first_paints_after_back_forward_cache_restore_swap_.size() - 1;
-  DCHECK(first_paints_after_back_forward_cache_restore_swap_[last_index]
-             .is_null());
-  first_paints_after_back_forward_cache_restore_swap_[last_index] = stamp;
+  DCHECK(!first_paints_after_back_forward_cache_restore_swap_.IsEmpty());
+  DCHECK(first_paints_after_back_forward_cache_restore_swap_.back().is_null());
+  first_paints_after_back_forward_cache_restore_swap_.back() = stamp;
   NotifyPaintTimingChanged();
 }