browser(firefox): introduce SimpleChannel

Review URL: aslushnikov/juggler@6364381

Refactor inter-process communication inside Firefox. The goal is
to have a single abstraction that works nicely for all our cross-process
communication needs (browser <-> content, content <-> workers, content
<-> file:// process, e.t.c.)

This is step 1 that eliminates content sessions everywhere.
Step 2 will move workers onto `SimpleChannel` as well.

This is a pre-requisite for microsoft#720: with a single `browser <-> content`
communication channel it will be easier to await permission change in tabs.

References microsoft#720

browser_patches/firefox/BUILD_NUMBER

@@ -1 +1 @@
-1032
+1033