[ChromeDriver] Minor fixes to Take Element Screenshot

Fixes a few minor issues in the implementation of Take Element
Screenshot command:
* Command display name was identical to whole window screenshot command.
* Command missing in client_replay.py.
* Function declaration was in the wrong header file.

Bug: chromedriver:1938
Change-Id: I2f263a150f801a70a32a1e0fca7ca6227e857d64
Reviewed-on: https://chromium-review.googlesource.com/1211245
Reviewed-by: Caleb Rouleau <crouleau@chromium.org>
Commit-Queue: John Chen <johnchen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#590600}
4 files changed