GitLab

Removed PresentationSessionDispatcher since the session messages are going through the
PresentationService (to reduce the number of Mojo pipes used).

Depends on the Blink change: https://codereview.chromium.org/1020303004

BUG=459006

Review URL: https://codereview.chromium.org/1024903003

Cr-Commit-Position: refs/heads/master@{#322162}