Bug 926105
Merge pull request #13232 from waalt/926105

Bug 926105 - When there are no apps to show in the app switcher, show a text saying "No recent apps", r=alive
Bug 921417 - rewrite build/variant.py in javascript
[music2] Fix the lint errors
Merge pull request #13159 from huchengtw-moz/build/Bug_921417_rewrite_variant.py_to_js

Bug 921417 - rewrite build/variant.py in javascript, r=yurenju, acperez
Merge pull request #13285 from dominickuo/bug-933110

Bug 933110 - [music2] Fix the jslint errors, r=@es92
Revert "Bug 921721 - hotfix to bust JS Exception in startup a=testonly r=me"

This reverts commit 5cf27888e390526c072d4b44a5af54c8aaf7205a.
Bug 923831 - Intermittent failures in test_kill_all, disabling the test until someone can investigate and/or repair the test
Bug 923831 - Intermittent failures in test_kill_all, fix manifest syntax a=testonly r=me