Currently resources that are modified between timing updates in page_timing_metrics_sender are held as a set of pointers to PageResourceDataUse objects. However, these are stored in a small map, which invalidates iterators/pointers on insertion, causing undefined behavior when using |modified_resources_|. 

See: https://logs.chromium.org/logs/chromium/buildbucket/cr-buildbucket.appspot.com/8935570890389859808/+/steps/browser_tests__with_patch_/0/logs/AutoplayMetricsBrowserTest.RecordAutoplayAttemptUkm/0 for one such example.