[chromedriver] Accept extensions in .zip format

BUG=chromedriver:776
TEST=chrome/test/chromedriver/test/run_py_tests.py --chromedriver=out/Release/chromedriver --filter=*ChromeExtensionsCapability*
(after building chromedriver: ninja -C out/Release chromedriver)

Review URL: https://codereview.chromium.org/248253002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@268432 0039d316-1c4b-4281-b951-d872f2087c98