Fix command handler for /session/:sessionId/execute (execute_script) for

Fix command handler for /session/:sessionId/execute (execute_script) for
ChromeDriver. The args parameter should be a ListValue, not a string.
Also, the args parameter is not optional; if no args are
provided in the request, an error should be returned (the handler was
defaulting to an empty list).
Patch by jleyba@chromium.org.
Original review at http://codereview.chromium.org/6544003
BUG=none
TEST=none

Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=75442