Precache.DownloadedPrecacheMotivated should account for the number of new bytes added to cache in every run.

However RecordStatsForFetchInternal() assumes that all requests are due to precache when precache task is running. This is not true. Frequently sync requests are sent when precache task starts, and this can lead to overcount the Precache.DownloadedPrecacheMotivated UMA.

https://cs.chromium.org/chromium/src/components/precache/content/precache_manager.cc?rcl=0&l=265