Add back the conservative page loading strategy

[shs96c]

This waits for all resources of a page to have finished
loading, and not just for the readyState to reach
"complete".

Addresses #893.

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This change is 

[shs96c]

Marking as an L2 fix. It's deemed to be a new feature, which is not in keeping with the CR period.

[jgraham]

Per the testing policy, this PR can't be accepted without a corresponding test in the web-platform-tests reposiotry.

[andreastt]

@shs96c Do you have time to also write the tests for this?

Because we don’t currently have any tests for the other page loading strategies it may be required to do those first, which could be a bit of work. @whimboo has written some excellent Marionette navigation tests that we would be happy to share with WPT, but we need to CC-0 license it first. @whimboo, would that be easy to do?

[whimboo]

@whimboo has written some excellent Marionette navigation tests that we would be happy to share with WPT, but we need to CC-0 license it first. @whimboo, would that be easy to do?

I wrote those tests as employee of Mozilla, so you can automatically handle those as CC-0. The tests can be found here:

https://hg.mozilla.org/mozilla-central/file/default/testing/marionette/harness/marionette_harness/tests/unit/test_navigation.py#l615