Bug 1411281 - Add equality test for webdriver.Element r=jgraham

When comparing two instances of webdriver.Element we want to first
check the type, to make sure the "id" attribute is present, then
compare the web element reference UUIDs.  These are supposed to be
unique across all browsing contexts.

MozReview-Commit-ID: 68PUBQxPdQ5

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extra : rebase_source : f0c52efd0db3a13ea1b90dc6c058ebee2de76e1d