`sender.frameId` should be set iff `sender.tab` is set, as documented at
https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/API/runtime/MessageSender
The removal of the `viewType == "tab"` condition broke this in
https://hg.mozilla.org/mozilla-central/rev/2dc4f1baccc8
This patch makes the presence of `frameId` conditional on `tab`, and
fixes several tests that relied on the incorrect behavior:
- Move the runtime.onConnect test from test_ext_contentscript_in_background.js
to a new mochitest at test_ext_runtime_connect_iframe.html.
- Simplify test_ext_contentscript_in_background.js to continue to
provide test coverage for contentScripts.register + allFrames.
- Replace runtime.onConnect with runtime.getFrameId in
test_ext_contentscript_xorigin_frame.js, since sender.frameId is no
longer available in xpcshell tests (because internals to support the
tabs extension API are not available in xpcshell tests). The test
cannot be moved to a mochitest because its purpose is to provide test
coverage for process switching in a xpcshell test (bug 1580811).
Differential Revision: https://phabricator.services.mozilla.com/D135057
c686f79b28