Support --additional-driver-flag in layout test wrapper.
Support --additional-driver-flag in layout test wrapper.

We supported the old --additional-drt-flag but not the newer
--additional-driver-flag.

R=carlosk@chromium.org

Review URL: https://codereview.chromium.org/1632043002

git-svn-id: 239fca9b83025a0b6f823aeeca02ba5be3d9fd76@298412 0039d316-1c4b-4281-b951-d872f2087c98

eunchong/build,eunchong/build,eunchong/build,eunchong/build