In  issue 782349 , I discovered that for an uninstalled extension, GetPrivilegeRequiredByUrl, used in ChromeContentBrowserClientExtensionsPart::IsSuitableHost, returns PRIV_EXTENSION for nonexistent extensions.  However, navigating to an uninstalled extension URL won't actually end up in a PRIV_EXTENSION process, because ChromeContentBrowserClientExtensionsPart::SiteInstanceGotProcess doesn't associate that process with the extension ID in the ProcessMap for uninstalled extensions.  This results in privilege mismatch between what we think is required for that URL and what we actually use.

It seems desirable to make these consistent by either (1) assigning PRIV_EXTENSION for nonexistent extensions in CCBCEP::SiteInstanceGotProcess, or (2) returning PRIV_NORMAL for nonexistent extensions from GetPrivilegeRequiredByUrl().  It's a little weird to mint a PRIV_EXTENSION process for an invalid extension URL, though maybe that consistency will help us avoid blocking CanCommitURL checks on this (i.e., we can still ensure that no chrome-extension:// URLs, even invalid ones, will commit in a PRIV_NORMAL process).

See more discussion on this at https://chromium-review.googlesource.com/c/chromium/src/+/762387/6/content/browser/renderer_host/render_process_host_unittest.cc#259.  I'm filing this to follow up on that.