Data from the UMA Sampling Profiler shows that content::BrowserContext::GetDownloadManager() regressed message loop startup on the browser process UI thread on 64-bit Chrome on Windows by 12ms. This occurred in the 71.0.3576.0 Canary release.

The responsible CL appears to be:
Reland "Download shelf : Fix for incognito"
7fa71af8fdd41ec3f43ba6e4c6c53e6488f119a6
author: shaktisahu@chromium.org

Execution profile difference of Canary 71.0.3575.0 vs. 71.0.3576.0: https://uma.googleplex.com/p/chrome/callstacks?sid=61145402a54f470049684ca576d5cd0c

The reason for the regression appears to be that the CL has caused the download manager UI to be unconditionally created at startup, rather than on-demand.