From September 13 on, pageviews with "unknown" referrer from Chrome Mobile dropped sharply, to almost a third of their previous level. (Similar on desktop Chrome, although there the absolute number was much smaller to begin with.)
It looks like this coincided with an increase in "external (search engine)" referrers on Chrome Mobile.
This change does not seem to have come from the rollout of a new Chrome Mobile version.
There are no corresponding entries in https://wikitech.wikimedia.org/wiki/Analytics/Data_Lake/Traffic/Webrequest#Changes_and_known_problems_since_2015-03-04 or related changelogs.