In bug 889220, we're working on making InputToFirstContentfulPaint have coverage for all page loads where there is user input. One thing uncovered in the analysis is that the metric currently doesn't include user inputs from Android intents, and it should.

LoadURLParams already has a mIntentReceivedTimestamp: https://cs.chromium.org/chromium/src/content/public/android/java/src/org/chromium/content_public/browser/LoadUrlParams.java?q=intentreceivedtimestamp&sq=package:chromium&dr=CSs&l=43

This is the time the intent was received, not the time of the user input. Not sure if it's possible to get the latter; if not, perhaps the former is the best we can do?