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Converts ChromeViewHostMsg_RunNetworkDiagnostics and
ChromeViewMsg_SetCanShowNetworkDiagnosticsDialg to mojom without
impacting message ordering.

The browser-side interface is exposed via a
WebContentsFrameBindingSet<T>.

The renderer-side (client) interface is exposed via
RenderFrameImpl's AssociatedInterfaceRegistry.

This CL serves as a simple example of converting routed IPC
messages which are handled by WebContentsObservers or other
WebContents-bound things.

BUG=612500

Review-Url: https://codereview.chromium.org/2310583002
Cr-Commit-Position: refs/heads/master@{#419259}