Update chromedriver to 2.12

----------ChromeDriver v2.12 (2014-10-27)----------
Supports Chrome v36-40
Resolved issue 946: ChromeDriver can't parse short git hashes from Chrome OS [Pri-0]
Resolved issue 906: Chromedriver cannot be instantiated on CrOS with **kwargs [OS-Chrome, Pri-1]
Resolved issue 944: When using mobile emulation, window.screen.height/width doesn't reflect phone's screen resolution [Pri-1]
Resolved issue 916: Selenium (javascript) fails when switching to webview window in Chrome apps [Pri-2]
Resolved issue 945: touch events sometimes don't work in mobile emulation mode [Pri-2]
Resolved issue 611: Prefs capability should override custom user data dir prefs [Pri-3]
Resolved issue 926: Chromedriver memory leak in devtools_client_impl.cc []