[chromedriver] Round dimensions when calculating element size

Improve detection of elements partially outside the viewport and return
their correct height.

When an element is only visible on the edge by between 0 and 0.5 the
WebRect casts the visible width/height to 0, causing an
ElementNotInteractable exception to be thrown with 'element has zero
size'.

This change updates the size calculation to round the intersection of
the window and element under test in the same way that is performed when
storing the Element Region into a WebRect object.

This has the effect that the getElementRegion() function falls back to
returning the clientRect size instead of the visible subregion when
calculating the visible portion.

R=johnchen@chromium.org, shengfa@google.com

Bug: chromedriver:3682
Change-Id: I5868c6d61ea56f5d229b0681f679670174dedf48
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2594837
Reviewed-by: Shengfa Lin <shengfa@google.com>
Reviewed-by: John Chen <johnchen@chromium.org>
Auto-Submit: Andrew Nicols <andrewrn@gmail.com>
Commit-Queue: Shengfa Lin <shengfa@google.com>
Cr-Commit-Position: refs/heads/master@{#838465}

• AUTHORS[diff]
• chrome/test/chromedriver/js/get_element_region.js[diff]
• chrome/test/chromedriver/js/get_element_region_test.html[diff]