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This is a reland of 70ffd447

Original change was reverted due to test flakiness on Mac, caused by:
* Pause time often goes above the upper limit of 300 ms, sometime more than
  double the limit.
* Occasionally the first event is missing.

The following changes are made to improve test reliability:
* Remove the check of an upper limit for the pause time. With the wide range
  of actual pause time, there appears to be no feasible upper limit to use.
* The first significant event is now pointer down instead of key down, to
  ensure that the target element receives focus. The previous dependence on
  autofocus attribute appears to be unreliable.
* Change mouse button from 1 (middle) to 0 (left), as the middle mouse button
  triggers paste on some platforms. (It was a typo.)

Original change's description:
> [ChromeDriver] Handle pauses in Perform Actions
>
> W3C spec allows the app to specify pauses within Perform Actions command
> (https://w3c.github.io/webdriver/#dfn-dispatch-actions). Updating
> ChromeDriver to handle these pauses.
>
> Bug: chromedriver:1897
> Change-Id: I533179f4a9f2216bfacf0f4fdb539c8f898bed07
> Reviewed-on: https://chromium-review.googlesource.com/c/1392281
> Commit-Queue: John Chen <johnchen@chromium.org>
> Reviewed-by: Caleb Rouleau <crouleau@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#619506}

Bug: chromedriver:1897
Change-Id: I38122bdc01965345421429b1d2f45374395b0cc7
Reviewed-on: https://chromium-review.googlesource.com/c/1394786Reviewed-by: Caleb Rouleau <crouleau@chromium.org>
Commit-Queue: John Chen <johnchen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#619717}