In certain situations, the logic to check when the iframe's document is
ready didn't work as expected. The reason for that is that browsers
create an initial blank document before the actual document loads.
This blank document has a `readyState` of 'complete' in Safari and
Chrome and of 'uninitialized' in Firefox. This blank document can be
mistaken by the final document. I was able to reproduce a situation
where the Hypothesis client was injected into the blank document. Then,
the blank document was replaced by the actual document and the
Hypothesis client was not not longer embedded.

In addition, this commit:

* renames `frame` to `iframe` (only in the test)
* renames `_handledFrames` to _annotatableFrames`
* removes `isAccessible` (no longer neede)