Add speedreader button via --enable-speedreader

[rebron]

Description

Add speedreader button via --enable-speedreader.
Remainder of speedreader feature outlined in #9810.

Can only view the speedreader button, states, and function with the command line above. Users should otherwise not see the speedreader button.

[karenkliu]

@AndriusA Designs added!

[petemill]

Should the speedreader ON page use a simple dark theme in dark mode? @karenkliu

[karenkliu]

@petemill Good Q! You're right, pages that are speed-readable in dark theme should have that styling carry over to the page content. I've updated the designs.

[iefremov]

For QA: The promo bubble was not implemented in this issue (there is a separate ticket #9262).

To enable speedreader pass --enable-speedreader. To whitelist certain sites use e.g. speedreader-whitelist="https://bbc.com/*;"

Specific details of the distilled page CSS are also out of this issue scope. Dark theme is not yet supported by the stylesheet.

[LaurenWags]

Using

Brave | 1.9.70 Chromium: 81.0.4044.138 (Official Build) (64-bit)
-- | --
Revision | 8c6c7ba89cc9453625af54f11fd83179e23450fa-refs/branch-heads/4044@{#999}
OS | macOS Version 10.14.6 (Build 18G3020)

Confirmed if no --enable-speedreader flag is used, there is no speedreader icon:

Confirmed if using --enable-speedreader flag, speedreader icon is shown:

Confirmed when icon is shown, I can interact with it (click on it).