GitLab

Introduce a mechanism for an incoming QuicAllowed cloud policy
to control QUIC protocol usage for new streams even after
IOThread initialization.
NetPrefObserver has been re-introduced to observe the change
(after it was removed in crrev.com/2140733002 and
crrev.com/2172543003).

BUG=658454
TEST=browser_tests --gtest_filter=*QuicAllowed*
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:closure_compilation

Review-Url: https://codereview.chromium.org/2546533003
Cr-Commit-Position: refs/heads/master@{#442866}