Bug 1058900 - [email] pinch/zoom mitigations

In some future, I wonder if it would make sense for the double tap logic to just set up a "scale factor" then call into a function that would do this work, so that there is just one set of newScale logic. I do not think it needs to be done now though, and it may not end up saving much, particularly if we ever do go back to fine-grained zoom. Sounds like that is most likely by going an apz-ed mozbrowser though, so it all may become moot at some point.

It does seem a bit odd though that double tap does not get a 1.5 option where pinching does, but can see that the double tap pathway was an existing one with just the base, 1 and 2 options.

Just ruminations based on being able to see the code after a few iterations. No need to change, given the hopeful future path where we do not manage this ourselves so much.