Re-enable fetching component policies for login screen apps

This CL relands enabling fetching of policies for Chrome OS login screen
apps.

The originally landed code was temporarily disabled due to DMServer
responding with errors when an unknown policy type was requested. Now
this can be enabled back, after the DMServer got fixed to at least
ignore the new policy type (until it starts to actually provide policies
with this type).

BUG=644304,666720
TEST=re-enabled browser tests;
     manual test: sign into an enrolled device, go to chrome://policy
     and check that the device policies were fetched successfully

Review-Url: https://codereview.chromium.org/2603463003
Cr-Commit-Position: refs/heads/master@{#441015}
4 files changed