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The devtools network tab receives a notification via "requestWillBeSent"
that corresponds to the time when the browser process queues a network
request. This enables the network tab to calculate how long the task
was queued.

The timeline (performance) panel does not receive a corresponding
tracing event, and hence does not know how long a task was queued. This
results in different times being reported as totals in the network vs
the performance panel if the task was initiated from the browser (and
not from the renderer).

This CL adds a tracing event to the browser that indicates that a
network request will be queued, and processes the event in DevTools
so that the network access time reported in the performance tab matches
the time reported in the network tab for requests initiated in the
browser.

For requests initiated in the renderer, the ResourceSendRequest tracing
event already has the time when the request gets queued (up to a few
1/100 ms).

Bug: chromium:865066
Change-Id: I1edfac4ec7ada8407b5dd931920e26e139b4f8df
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1771398
Commit-Queue: Sigurd Schneider <sigurds@chromium.org>
Reviewed-by: Alexei Filippov <alph@chromium.org>
Cr-Commit-Position: refs/heads/master@{#695614}