Record send_start timing info after starting ServiceWorker
The send_start field of LoadTimingInfo, which corresponds to the
requestStart attribute of Resource Timing [1], was recorded before
starting ServiceWorker. Since that point should be represented as the
workerStart attribute [2], this patch moves send_start to
DidPrepareFetchEvent, that is just before dispatching fetch event.
[1] https://w3c.github.io/resource-timing/#widl-PerformanceResourceTiming-requestStart
[2] https://w3c.github.io/resource-timing/#widl-PerformanceResourceTiming-workerStart
BUG=465640
TEST=n/a
Review URL: https://codereview.chromium.org/
1155043002
Cr-Commit-Position: refs/heads/master@{#331271}