Isolate shell operations to the utility process.

Shell operations can cause 3rd-party shell extensions to be
loaded into the caller's process. The utility process protects the browser process from potential instability.

Also convert the IPC mechanism to Mojo.

BUG=73098

Review-Url: https://codereview.chromium.org/2122303002
Cr-Commit-Position: refs/heads/master@{#405270}
16 files changed