Add a user agent stylesheet for extension options pages
Apply WebUI styles to extension options pages that opt into using them
through a user agent stylesheet. The styles are copied from widgets.css
and chrome_shared.css in ui/webui/resources/css/ - this is intentional
so that WebUI changes won't break extensions.
The extension.css stylesheet will only be applied to extensions that
opt in using the "options_ui.chrome_style" manifest entry, and it will
only affect the options page specified in the manifest. The font
stylesheet for extensions have been moved to extension_fonts.css, and
are still applied to every extension page.
BUG=408271
Review URL: https://codereview.chromium.org/
537773003
Cr-Commit-Position: refs/heads/master@{#294491}