White flash after tab open or close

[petemill]

Test plan

See #11818

Description

When opening a tab, there is a flash of the background of the <webview> component, which is explicitly set to white in window.less.
When closing a tab, there is a flash of the background of the whole window (or at least div.tabContainer) as the active frame is removed, but the next active frame is not set active yet.

The opening tab scenario can be solved by setting the <webview> background to not be white in certain cases, or not showing it until the background of the incoming page is ready (perhaps, more risky).

the closing tab scenario can be solved by either:

1. Setting next active tab before closing the tab, and then waiting Xms to actually close the tab (tested it and works OK, but the browser process feels the wrong place to do it for a rendering issue), or;
2. Keeping the frame (and <webview>) around until the newly active frame has a chance to be shown (tested and works OK, but an arbitrary number of ms)

Steps to Reproduce

Set new tab page to Dashboard.
Open and close many tabs.

Actual result:
Observe white flashes

Expected result:
No white flashes between opening or closing tabs

Reproduces how often:
100%

Brave Version

about:brave info:
master

Reproducible on current live release:
Yes

Additional Information

PR incoming