Fix crash on It2Me host.
Fix crash on It2Me host.

This is a regression from https://codereview.chromium.org/1530523002.
We should be passing signal_strategy_.get() instead of signal_strategy.get()
to the underlying JingleSessionManager in
it2me_host.cc

BUG=579758

Review URL: https://codereview.chromium.org/1638733002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#371452}

axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk