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Here "ResourceHandler" refers to the NavigationResourceHandler or the
MojoAsyncResourceHandler depending on whether NavigationMojoResponse is
enabled or not.

It adds two histograms:
* Navigation.ResourceHandler.ResponseStartedUntilProceedWithResponse
* Navigation.ResourceHandder.ProceedWithResponseUntilStartLoadingResponseBody.

It records the time delta between these 3 events.
1) OnResponseStarted().
2) ProceedWithResponse().
3) OnReadCompleted() is called for the first time.

Bug: 705744
Change-Id: I10e26016df13b9f9a7c0687774986f8775ce59c3
Reviewed-on: https://chromium-review.googlesource.com/937516
Commit-Queue: Ilya Sherman <isherman@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Cr-Commit-Position: refs/heads/master@{#539982}