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I'd like to use Mojo to send navigation preload response (crbug.com/649558) from
the browser process to the service worker. But currently we are using the legacy
IPC when the browser process sends the FetchEvent to the service worker. There
will be an ordering issue if we use both the legacy IPC and Mojo IPC.

To avoid the ordering issue, this CL changes the FetchEvent IPC to use Mojo.

BUG=629701,649558

Review-Url: https://codereview.chromium.org/2352173004
Cr-Commit-Position: refs/heads/master@{#422996}