Specify an opt in to enable commands that rely on the accessibility tree

[zcorpan]

#1441 and #1439 need the accessibility tree to be available. @cookiecrook pointed out that it may need an opt-in instead of being always available because in browsers today the accessibility tree is only built when it is needed, since there is a performance cost.

So this opt-in should be decided on and specified.

cc @spectranaut @jugglinmike @AutomatedTester @alice

[gsnedders]

Is this an opt-in that can be enabled after page-load, or does it need to be enabled prior?

Other things we're possibly going to support (like some limited debug functionality) is also going to require opt-in, so we should probably ensure we have some generic opt-in mechanism.