Bug 1398499: Part 2 - Add MOZ_LOADER_SHARE_GLOBAL env var to toggle global sharing. r=mccr8

User preference changes currently don't reliably take effect before component
loader initialization, which means they can't be used to write reliable tests.
Environment variables don't have this problem, so adding an environment
variable override makes testing much easier. It's also fairly convenient
during development, when we need to switch between different configurations
for testing.

MozReview-Commit-ID: 8PufRQNRnoU

--HG--
extra : rebase_source : c5ca2f3cb18a8398c95bbbf86e2cd27430f5161a