rMOZILLARELEASE80af7dfd0107

Bug 1616684. Remove implicit integer conversion in CheckedInt constructor r=froydnj

When I try to enable -Wconversion, CheckedInt triggers it because it has an
implicit type conversion in its templated two-arg constructor.

That constructor doesn't really need to be templated, as far as I can tell.

Differential Revision: https://phabricator.services.mozilla.com/D63384