GitLab

Moved flags initialization from FakePortAllocatorSession into FakePortAllocator,
so that they are passed to the PortAllocatorSession ctor rather than being set via
set_flags() in the FakePortAllocatorSession ctor. This is required because some
flags (specifically PORTALLOCATOR_ENABLE_SHARED_UFRAG) are ignored if applied to
PortAllocatorSession via set_flags().

Review URL: https://codereview.chromium.org/1143893002

Cr-Commit-Position: refs/heads/master@{#330875}