Update client side navigator.connect API to use ServicePortCollection [2/3]

Update client side navigator.connect API to use ServicePortCollection [2/3]

This is part of a series of patches to update the client side API of what was
formerly known as navigator.connect to follow the new spec based on
navigator.services and a new ServicePort type.

This patch updates the content side code to use mojo for most of its IPC,
although still relying somewhat on MessagePort since the existing service side
code uses MessagePort concepts. A future patch series will also adjust the service
side API to use ServicePort and get rid of the dependency on MessagePort.

[1/3] https://codereview.chromium.org/1191393003 Blink side changes
[2/3] https://codereview.chromium.org/1192713004 This patch
[3/3] https://codereview.chromium.org/1198653004/
      Cleanup and removing old code paths on the blink side and in tests.

BUG=426458

Review URL: https://codereview.chromium.org/1192713004

Cr-Commit-Position: refs/heads/master@{#336636}