Spelling cannot cache words across invocations.

We want to let the facilitator decide if a word is valid or invalid, and cache
the answer in the facilitator's cache. The spell checker session doesn't need
its own word cache, except as a crutch to communicate suggestions to the code
that populates the suggestion drop-down. We leave that in place.

Bug 20018546.

Change-Id: I3c3c53e0c1d709fa2f64a2952a232acd7380b57a