Pass "client_id" param to the enrollment frame.
A "client_id" param passed to GAIA should match a "client_id" that is used for
token exchange afterwards, otherwise token exchange doesn't work. If the param
is not specified, GAIA uses default value, which is the same as official Google
Chrome's "client_id".
We didn't provide "client_id" for enrollment frame, as a result enrollment only
worked in official builds.
BUG=495527
TEST=browser_tests
Review URL: https://codereview.chromium.org/
1282393002
Cr-Commit-Position: refs/heads/master@{#342871}