The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/f954b63b01f228cf352d7b0294c291d24a1e9249

commit f954b63b01f228cf352d7b0294c291d24a1e9249
Author: Matt Falkenhagen <falken@chromium.org>
Date: Wed Jun 14 23:02:47 2017

service worker: Migrate away from _StartWorkerExistingProcess metrics

These metrics are obsolete in the post-PlzNavigate world. Pre-PlzNavigate,
_StartWorkerExistingProcess was the common case for navigations,
because navigation and service worker startup would start after the renderer
process launches and issues the request for the URL. Therefore, we
targeted SWEP as the common case to optimize and care about.

However, post-PlzNavigate, _StartWorkerNewProcess is the common case, and
_StartWorkerExistingProcess is also fairly common.

Move the metrics to log to _WorkerStartOccurred, which is logged whenever the
worker was not already in status RUNNING. This is wider than the union of
_StartWorkerExistingProcess and _StartWorkerNewProcess, since it also includes
_DuringStartup as well as cases where the worker was originally in status
STOPPING or STARTING. But in practice, these other cases are rare and this
strikes a good balance of not creating a histogram for every possible state
while capturing what we really care about: whether the worker was already
running or not.

Bug:  733070 
Change-Id: If3aa99e6d83e6b47d6a81d2cc37fb6922e36fa97
Reviewed-on: https://chromium-review.googlesource.com/535374
Reviewed-by: Tsuyoshi Horo <horo@chromium.org>
Reviewed-by: Jesse Doherty <jwd@chromium.org>
Commit-Queue: Matt Falkenhagen <falken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#479535}
[modify] https://crrev.com/f954b63b01f228cf352d7b0294c291d24a1e9249/content/browser/service_worker/service_worker_metrics.cc
[modify] https://crrev.com/f954b63b01f228cf352d7b0294c291d24a1e9249/content/browser/service_worker/service_worker_metrics.h
[modify] https://crrev.com/f954b63b01f228cf352d7b0294c291d24a1e9249/content/browser/service_worker/service_worker_metrics_unittest.cc
[modify] https://crrev.com/f954b63b01f228cf352d7b0294c291d24a1e9249/content/browser/service_worker/service_worker_url_request_job.cc
[modify] https://crrev.com/f954b63b01f228cf352d7b0294c291d24a1e9249/content/browser/service_worker/service_worker_url_request_job_unittest.cc
[modify] https://crrev.com/f954b63b01f228cf352d7b0294c291d24a1e9249/tools/metrics/histograms/histograms.xml

We're pretty good here now, there weren't as many as I thought.