Any JS code (including comments) gets loaded into content process memory
for every single content process that is loaded. This patch tries to
reduce this burden as much as possible by moving things into either the
TranslationsParent, or into lazily loaded modules in the child.
This patch also changes the engine caching logic to only allow 1 engine
cache at a time. I was struggling porting over the existing cache to this
strategy, and this felt like a much simpler solution.
Differential Revision: https://phabricator.services.mozilla.com/D180482
c2e8fe37ab