Don't create a pairing registry on platforms that don't provide a delegate.
Don't create a pairing registry on platforms that don't provide a delegate.

This was broken by https://codereview.chromium.org/834673007. Constructing
a PairingRegistry with a null delegate causes pairing to be offered, but if
selected, the host process will crash.

BUG=455501
TBR=jamiewalch@chromium.org

Review URL: https://codereview.chromium.org/903963002

Cr-Commit-Position: refs/heads/master@{#314969}
(cherry picked from commit dbec99efb805e22b77b5c2d18a2f21e405fbc5cf)

Review URL: https://codereview.chromium.org/889493004

Cr-Commit-Position: 665a5787e4610f1ec55d3a4e2f02ab0724e38987@{#233}
Cr-Branched-From: 4cd5f18d847990599feba509541d8a7b9a16883a@{#310958}

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