opening Incognito, Tor and Guest windows for the first time appears janky when loading "brave://newtab"

[kjozwiak]

Description

After implementing the new brave://newtab pages, opening Incognito, Tor or Guest windows for the first time will appear white for a few seconds before loading brave://newtab. Sometimes this makes the window appear as it's "flashing". Sometimes it loads pretty quickly and sometimes there's a delay which makes it look like brave is janky/having issues loading the window.

We should make this a smoother transition.

Steps to Reproduce

1. install & open 0.55.16 Chromium: 70.0.3538.54 (release)
2. open Incognito, Tor or Guest windows

You'll notice that when the window initially loads, it will appear white for a few seconds before loading brave://newtab.

Actual result:

https://youtu.be/Rrmo6gGqq80

Expected result:

Opening Incognito, Tor and Guest windows should be a smooth experience.

Reproduces how often:

100% reproducible using the above STR.

Brave version (brave://version info)

Brave	0.55.16 Chromium: 70.0.3538.54 (Official Build) (64-bit)
Revision	4f8e578b6680574714e9ed3bb9f02922b4dde40d-refs/branch-heads/3538@{#937}
OS	Mac OS X

Reproducible on current release:

• Does it reproduce on brave-browser dev/beta builds?

Not 100% sure as the changes just landed and we don't have dev nor beta builds available that include these new changes.

• Does it reproduce on browser-laptop?

N/A

[GeetaSarvadnya]

The issue is Reproducible in windows 10 x64 - Brave beta0.70.109. White window flash is more Prominent before loading Guest/Tor window