mojo: Strongly bind WebUI handler to pipe.

The same WebUI controller can be used by multiple pages. As of now, each
request for a handler would delete the previous one. This CL change the
lifetime of the handle by binding those to the pipe.

R=sammc@chromium.org

Review URL: https://codereview.chromium.org/630923002

Cr-Commit-Position: refs/heads/master@{#298677}
3 files changed