Avoid copying by using 'find()' instead of 'at()'

Bug: 943732
Change-Id: I7b8566db4dfd7e62c369f685f0d21c1644ce1cc1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2050607Reviewed-by: Yoav Weiss <yoavweiss@chromium.org>
Commit-Queue: Tom McKee <tommckee@chromium.org>
Cr-Commit-Position: refs/heads/master@{#741468}

third_party/blink/renderer/core/timing/performance_user_timing.cc

@@ -114,8 +114,11 @@ void UserTiming::ClearMarks(const AtomicString& mark_name) {
 
 double UserTiming::FindExistingMarkStartTime(const AtomicString& mark_name,
                                              ExceptionState& exception_state) {
-  if (marks_map_.Contains(mark_name))
-    return marks_map_.at(mark_name).back()->startTime();
+  PerformanceEntryMap::const_iterator existing_marks =
+      marks_map_.find(mark_name);
+  if (existing_marks != marks_map_.end()) {
+    return existing_marks->value.back()->startTime();
+  }
 
   PerformanceTiming::PerformanceTimingGetter timing_function =
       PerformanceTiming::GetAttributeMapping().at(mark_name);