[chromedriver] Rename "emulateViewport" to "mobile".

BUG=chromedriver:843
TBR=stgao@chromium.org

Review URL: https://codereview.chromium.org/379803007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@282089 0039d316-1c4b-4281-b951-d872f2087c98