Fixes facebook#13777

As part of facebook#11927 we introduced a regression by adding onclick handler
to the React root. This causes the whole React tree to flash
when tapped on iOS devices (for reasons I outlined in facebook#12989 (comment)).

To fix this, we should only apply onclick listeners to portal roots. I
verified that this fix indeed worked by checkout out our DOM fixtures
and added regression tests as well.

Strangely, I had to make changes to the DOM fixtures to see the behavior
in the first place. This seems to be caused by our normal sites being
bigger than the viewport:

![](http://cl.ly/3f18f8b85e91/Screen%20Recording%202018-10-05%20at%2001.32%20AM.gif)

An alternative fix would be to add a third parameter to
`appendChildToContainer` based on the tag of the parent fiber. Although
I think relying on the `_reactRootContainer` property that we set on the
element is less intrusive.