[Chromedriver] Add Device Metrics override support to ChromeDriver via Capabilities

BUG=chromedriver:399

All of the code reviews for this patch were written against https://codereview.chromium.org/251933005/ . I used the wrong email address to create said patch, so I have created this one with the correct email address.

Review URL: https://codereview.chromium.org/288193004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@271947 0039d316-1c4b-4281-b951-d872f2087c98