Make VR screen tap tests more stable

The single tap sent during the VR screen tap tests wasn't always
getting registered since device response can be a bit slow during
the VR transition. For the Cardboard case, wait on Javascript to say
it received another click event. For the Daydream case, wait on VrShell
saying its parent has consumed the event before checking.
BUG=

Review-Url: https://codereview.chromium.org/2768583002
Cr-Commit-Position: refs/heads/master@{#460452}
7 files changed