[iOS] CWTChromeDriver: Add support for screenshots

This CL adds support for taking a snapshot of the target tab. It also
adds a no-op implementation of the "Set Window Rect" WebDriver
command. These operations are used by the Web Platform Test runner when
running reftests (tests that load two pages, take snapshots of both, and
then check if the snapshots are identical).

Bug: 673423
Change-Id: Icda78a3c81ba499e2ef05ecb98c3c771da5d1dd0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1689502
Commit-Queue: Ali Juma <ajuma@chromium.org>
Reviewed-by: Rohit Rao <rohitrao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#677343}