Small JS code cleanup.

1. `cat /proc/<pid>/cpuset` returns "background" for EJs, which implies
   they'll run on little cores
2. Added some annotations
3. Removed specialized method in favor of generic method.

Bug: 141645789
Bug: 171305774
Test: Android builds
Change-Id: I4b6a5776eddbcec1bc28ebbd0ba38577ba3a93c3