Merge (browser_style) extension-mac.css with extension.css and use
-moz-platform media query for conditional CSS.
Replace extension-{mac,linux,win}-panel.css with one CSS file, without
platform-specific conditions since their content were already identical.
Simplified JS code referencing these CSS files: Instead of lazy getters
("extensionStylesheets") exported and imported across modules, use plain
string constants for simplicity. The extension.css file is not expected
to be used in new code since it is a deprecated feature, and the string
is sufficiently unique to be easily findable through code search.
Differential Revision: https://phabricator.services.mozilla.com/D198312
ac7632657a