[ChromeDriver] Add Pixel 2 to mobile emulation list

Also modified embed_mobile_devices_in_cpp.py, so users can use both old
device names such as 'iPhone 6', and new names such as 'iPhone 6/7/8'.

Bug: chromedriver:2231
Change-Id: I476527d223bd6d3e440bef3823edba65b214b1fd
Reviewed-on: https://chromium-review.googlesource.com/879467
Reviewed-by: Jonathon Kereliuk <kereliuk@chromium.org>
Commit-Queue: John Chen <johnchen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#534553}