Tabs which start at about: urls (like new tab) won't ever be automatically discarded

[petemill]

#13120 fixes and re-introduces tab discarding, with the same logic as previous:

• If a tab is created with an about url, even about:newtab, then we set autoDiscardable: false in createProperties.
  The problem here is that some or many or most user tabs will be created with the starting url of about:newtab or about:blank, which will render the tab discarding feature much less useful if this logic remains.

The difficulty to reproduce this is what led to this becoming a hidden bug until now: We created a manual tab discard debug option, which is not impacted by this logic. Automatic tab discarding is affected, and is the purpose of the tab discarding feature.

I will add a logging flag in a potential fix in order to observe when a tab has been discarded.

Test plan / Steps to Reproduce

It is very difficult to reproduce, since one must reproduce with the same conditions neccessary to force the underlying browser to delete a tab's contents due to memory pressure. I have had some luck with the following, on a Windows VM allocated a smaller amount of memory than usual:

1. Create a new tab (starting at the default about:newtab homepage)
2. Cause memory pressure (create many many new tabs, with many open apps, and make those tabs active 1 by 1, rapidly)

Actual result:

Tab is not discarded (does not reload when visited)

Expected result:
Tab is discarded when not active for some time (reloads when visited). Unfortunately we do not have the equivalent of chrome://discards to observe when a tab has been automatically discarded.

Reproduces how often:

Brave Version

about:brave info:
0.22.x / master

Potential Solutions

1. Call webContents.setAutoDiscardable(bool) when navigation to a new URL is complete, and follow existing isHttpOrHttps || pinned logic.
2. Allow tabs which start at about:... URLs to be auto discardable. (but what about pinned vs non-pinnned?)

[bsclifton]

Marked as QA/no-qa-needed because the test plan would be the same as regular tab discarding (but for about pages)