Replace ellipsis with fade-out for tab title

[cezaraugusto]

Auditors: @ayumi

Fix #7535

cc @bradleyrichter

Test plan:

1. Have a page open that has themeColor: i.e. https://brianbondy.com, and another with no themeColor: i.e. about page
2. Should be aesthetically OK on tab's active/inactive states.
3. Should be aesthetically OK when resized
4. Should be aesthetically OK on private/new session tabs

[ayumi]

this is minor but I'd like a more subtle fade like

[cezaraugusto]

ok updated

[ayumi]

minor: for me private tabs when first opened have white text on white background

[ayumi]

besides minor comment lgtm

[bsclifton]

Awesome work! This may be a small change, but I feel like it polishes up Brave quite a bit. I checked against Chrome and the behavior seems to match EXCEPT (not related to this PR) the close button is always shown for tabs in Chrome.

@bradleyrichter do we want to consider always showing the X for the close button?