[ChromeDriver] Implementation of GetElementRect

Spec is here:
https://w3c.github.io/webdriver/webdriver-spec.html#get-element-rect


Bug: chromedriver:1937
Change-Id: I6b035a9f9526fdb5b3ba29c7c0a7269086216d50
Reviewed-on: https://chromium-review.googlesource.com/963044
Commit-Queue: Jonathon Kereliuk <kereliuk@chromium.org>
Reviewed-by: John Chen <johnchen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#543429}
7 files changed