Fix paper tooltip fit to visible bounds.

[jpalmer]

There are 3 cases that can happen with fit to visible bounds (Using left/right for simplicity here)

1. Off the right of the screen
2. Off the left of the screen
3. In the middle of the screen (no adjustment required)

Paper tooltip failed to handle case (3).

Fix that.

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[bicknellr]

Imported for testing as http://cl/384586759. I think our test infra for everything in https://github.com/PolymerElements broke recently due to travis-ci.org being turned down so I'll have to try this out locally.

[jpalmer]

Friendly ping :)

[jpalmer]

ping