hidden-content check - Shadow DOM support

[marcysutton]

Failing tests for the hidden-content check:

    35) should return undefined with display:none and children
    36) should return undefined with visibility:hidden and children
    37) should return true with visibility:hidden and parent with visibility:hidden
    38) should return true with aria-hidden and no content

Subtask of #87.

Full stack trace:

35) hidden content should return undefined with display:none and children:
     
  hasContent@http://localhost:9876/axe.js:8651:27
  evaluate@http://localhost:9876/tmp/rules.js:2076:85
  http://localhost:9876/test/checks/visibility/hidden-content.js:21:60
  callFn@http://localhost:9876/node_modules/mocha/mocha.js:4447:25
  run@http://localhost:9876/node_modules/mocha/mocha.js:4439:13
  runTest@http://localhost:9876/node_modules/mocha/mocha.js:4936:13
  http://localhost:9876/node_modules/mocha/mocha.js:5042:19
  next@http://localhost:9876/node_modules/mocha/mocha.js:4853:16
  http://localhost:9876/node_modules/mocha/mocha.js:4863:11
  next@http://localhost:9876/node_modules/mocha/mocha.js:4787:16
  http://localhost:9876/node_modules/mocha/mocha.js:4831:9
  timeslice@http://localhost:9876/node_modules/mocha/mocha.js:82:27

  36) hidden content should return undefined with visibility:hidden and children:
     
  hasContent@http://localhost:9876/axe.js:8651:27
  evaluate@http://localhost:9876/tmp/rules.js:2076:85
  http://localhost:9876/test/checks/visibility/hidden-content.js:27:60
  callFn@http://localhost:9876/node_modules/mocha/mocha.js:4447:25
  run@http://localhost:9876/node_modules/mocha/mocha.js:4439:13
  runTest@http://localhost:9876/node_modules/mocha/mocha.js:4936:13
  http://localhost:9876/node_modules/mocha/mocha.js:5042:19
  next@http://localhost:9876/node_modules/mocha/mocha.js:4853:16
  http://localhost:9876/node_modules/mocha/mocha.js:4863:11
  next@http://localhost:9876/node_modules/mocha/mocha.js:4787:16
  http://localhost:9876/node_modules/mocha/mocha.js:4831:9
  timeslice@http://localhost:9876/node_modules/mocha/mocha.js:82:27

  37) hidden content should return true with visibility:hidden and parent with visibility:hidden:
     
  hasContent@http://localhost:9876/axe.js:8651:27
  evaluate@http://localhost:9876/tmp/rules.js:2076:85
  http://localhost:9876/test/checks/visibility/hidden-content.js:33:55
  callFn@http://localhost:9876/node_modules/mocha/mocha.js:4447:25
  run@http://localhost:9876/node_modules/mocha/mocha.js:4439:13
  runTest@http://localhost:9876/node_modules/mocha/mocha.js:4936:13
  http://localhost:9876/node_modules/mocha/mocha.js:5042:19
  next@http://localhost:9876/node_modules/mocha/mocha.js:4853:16
  http://localhost:9876/node_modules/mocha/mocha.js:4863:11
  next@http://localhost:9876/node_modules/mocha/mocha.js:4787:16
  http://localhost:9876/node_modules/mocha/mocha.js:4831:9
  timeslice@http://localhost:9876/node_modules/mocha/mocha.js:82:27

  38) hidden content should return true with aria-hidden and no content:
     
  hasContent@http://localhost:9876/axe.js:8651:27
  evaluate@http://localhost:9876/tmp/rules.js:2076:85
  http://localhost:9876/test/checks/visibility/hidden-content.js:39:55
  callFn@http://localhost:9876/node_modules/mocha/mocha.js:4447:25
  run@http://localhost:9876/node_modules/mocha/mocha.js:4439:13
  runTest@http://localhost:9876/node_modules/mocha/mocha.js:4936:13
  http://localhost:9876/node_modules/mocha/mocha.js:5042:19
  next@http://localhost:9876/node_modules/mocha/mocha.js:4853:16
  http://localhost:9876/node_modules/mocha/mocha.js:4863:11
  next@http://localhost:9876/node_modules/mocha/mocha.js:4787:16
  http://localhost:9876/node_modules/mocha/mocha.js:4831:9
  timeslice@http://localhost:9876/node_modules/mocha/mocha.js:82:27