And remove redundant `Promise.resolve()` because it is the default
for async functions.
setIcon is not supported on Android, so there was no need to change
mobile/android/components/extensions/ext-pageAction.js.
MozReview-Commit-ID: 94ebaJFxLAi
--HG--
extra : rebase_source : 20466181501b264ba33fc8ab61fdf2bed20f9eef
