The TabStats field |is_auto_discardable| is never set.[1]

TabManager::CompareTabStats() looks at that field first, so it's the most important from the tab importance perspective. When tabs are sorted by importance, non-auto-discardable tabs should be at the top of the list in chrome://discards. This is not the case. This can be tested by using an extension to prevent auto-discard for a tab and seeing that tab's location in the list at chrome://discards.

However, the corresponding |is_auto_discardable| field in TabManager::WebContentsData::Data *is* set, and that's the version used to check whether a tab can be discarded in TabManager::CanDiscardTab(). So this doesn't lead to a runtime issue: An extension can protect a tab via chrome.tabs.update(), and that tab will correctly be protected from auto-discarding.

Here are two possible fixes:

1. Set |is_auto_discardable| in TabManager::AddTabStats() using
   GetWebContentsData(contents)->IsAutoDiscardable().
2. Drop |is_auto_discardable| from TabStats and do not use it in
   TabManager::CompareTabStats(). It's already enforced via
   TabManager::CanDiscardTab().

As a general question, when should a field in TabManager::WebContentsData::Data have a copy in TabStats?

[1] https://cs.chromium.org/search/?q=is_auto_discardable&sq=package:chromium&type=cs