Drive out additional flakiness of TouchAction browser test.

It is relatively stable but it has flaked a couple of times specifically
with this:

  Actual: 44
Expected: distance.y()
Which is: 45

I presume that the failure is either we aren't waiting for the additional
frame or a subpixel scrolling problem. As a speculative fix wait for the
pixel item we are waiting to scroll for.

BUG=376668

Review-Url: https://codereview.chromium.org/2281613002
Cr-Commit-Position: refs/heads/master@{#414525}
  TouchActionBrowserTest() {}

  TouchActionBrowserTest() {}
