Record send_start timing info after starting ServiceWorker

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

Committed: https://crrev.com/a14b3794ee2e759cae74a48d8554fad69396577c
Cr-Commit-Position: refs/heads/master@{#331271}

[Kunihiko Sakamoto, 2015-05-25 08:04:37]

ksakamoto@chromium.org changed reviewers:
+ horo@chromium.org

[Kunihiko Sakamoto, 2015-05-25 08:04:37]

ptal

[horo, 2015-05-25 08:40:30]

lgtm

[Kunihiko Sakamoto, 2015-05-25 08:42:24]

The CQ bit was checked by ksakamoto@chromium.org

[commit-bot: I haz the power, 2015-05-25 08:42:33]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1155043002/1

[commit-bot: I haz the power, 2015-05-25 09:45:49]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-05-25 09:46:43]

Patchset 1 (id:??) landed as
https://crrev.com/a14b3794ee2e759cae74a48d8554fad69396577c
Cr-Commit-Position: refs/heads/master@{#331271}

[Kunihiko Sakamoto, 2015-05-26 01:18:03]

Patchset #2 (id:20001) has been deleted