ExtensionContext in Extension.jsm has |extension| as an instance member,
so use it instead of passing |extension| to registerSchemaAPI's
callback.
And to make sure that this pattern also works in content processes, move
the |extension| member to BaseContext.
MozReview-Commit-ID: BgsGGCPQxJR
The base .eslintrc is essentially a merge of the root Toolkit .eslintrc and
the devtools .eslintrc, with some minor changes to match our prevalent style.
For the most enforces the coding styles that we've been using most
consistently. There are a couple of significant differences, though:
* The rule for opening brace alignment can only be applied globally, and
doesn't make exceptions for top-level functions. I chose to turn it on, and
change the brace style of existing top-level functions that violated it,
since the rule seemed worth using, and that's the direction most Toolkit JS
code has been headed anyway.
* The rule for switch/case statements requires an added indentation level for
case statements. Most of our switch statements did not use an extra level
of indentation, and I initially wrote the rule to enforce that style, until
I came across case statements that used blocks, and required the extra
indentation level for sanity.
