[ChromeDriver] Implementation of Take Element Screenshot

Also involves changing the function signature and logic a bit
around web_view CaptureScreenshot.

spec: https://w3c.github.io/webdriver/webdriver-spec.html#dfn-take-element-screenshot


(this CL is created based on https://chromium-review.googlesource.com/c/chromium/src/+/984343)

Bug:chromedriver:1938

Change-Id: Ibd3f57ae3aa71ae65d315a9066d3f2f490887d61
Reviewed-on: https://chromium-review.googlesource.com/1194247
Reviewed-by: John Chen <johnchen@chromium.org>
Commit-Queue: Tatiana Buldina <buldina@chromium.org>
Cr-Commit-Position: refs/heads/master@{#586877}
11 files changed