Add commands to manage tabs and contexts to Browser Domain

Browser.newPage and Browser.closePage are usable in both normal chrome
and in headless chrome.  Browser.newContext and Browser.closeContext
are implemented in headless chrome only.

Note Browser.newPage has a number of headless chrome only
parameters: width, height, contextId

BUG=546953

Review-Url: https://codereview.chromium.org/2119063002
Cr-Commit-Position: refs/heads/master@{#404099}
22 files changed