Fix PepperPortAllocatorSession to initialize callback_factory_.

After crrev.com/368701 PepperPortAllocatorSession constructor wasn't
initializing calllback_factory_ properly, so the plugin was crashing.

BUG=576985

Review URL: https://codereview.chromium.org/1584883002

Cr-Commit-Position: refs/heads/master@{#369245}
1 file changed