Fix event page process cleanup when running in --isolate-extensions mode.

With --isolate-extensions, navigations can start in one RenderFrameHost
and complete in another. This was not possible to happen with extensions
related RenderFrameHosts in the past, so the code wasn't able to handle
it. This CL refactors the OnNetworkRequest(Started|Stopped) to use the
navigation request id as the stable identifier instead of the associated
RenderFrameHost.

BUG=612668

Review-Url: https://codereview.chromium.org/2028873002
Cr-Commit-Position: refs/heads/master@{#397328}
5 files changed