BackgroundSync sync events need to be ExtendableEvents, content side

BackgroundSync sync events need to be ExtendableEvents, content side

This is part of a series of patches to correctly fire the Sync event as
an ExtendableEvent, instead of a regular Event

[1/4] This patch, content side fix
[2/4] https://codereview.chromium.org/1148293011/, blink side fix
[3/4] https://codereview.chromium.org/1160253004/, content browser tests
[4/4] https://codereview.chromium.org/1146063005/, cleanup

Specific changes in this patch
- Sync event finished handling accepts the result of the waitUntil promise
- The worker status will be set correctly if the promise was rejected
- Overload of existing public/web interface provided for compatibility
  with blink implementation

BUG=486890
Committed: https://crrev.com/b2bc492b3a8709bd3398cf4ea18ddc1909286d2d
Cr-Commit-Position: refs/heads/master@{#332787}

[chasej, 2015-06-02 17:42:51]

chasej@chromium.org changed reviewers:
+ jkarlin@chromium.org

[chasej, 2015-06-02 17:43:35]

Please take a look, and recommend other reviewers.

[jkarlin, 2015-06-03 14:59:06]

Nice, lgtm. Suggest adding tsepez for service_worker_messages.h and michaeln for
the rest.

[chasej, 2015-06-03 15:39:14]

chasej@chromium.org changed reviewers:
+ michaeln@chromium.org, tsepez@chromium.org

[chasej, 2015-06-03 15:42:57]

tsepez, please take a look at service_worker_messages.h.

michaeln, please take a look at the rest of the changes (under browser/ and
renderer/)

[Tom Sepez, 2015-06-03 19:43:02]

Messages LGTM

[michaeln, 2015-06-04 03:00:59]

lgtm 2

[chasej, 2015-06-04 04:53:35]

The CQ bit was checked by chasej@chromium.org

[commit-bot: I haz the power, 2015-06-04 04:53:45]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1162243003/20001

[commit-bot: I haz the power, 2015-06-04 05:53:11]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-06-04 05:54:20]

Patchset 2 (id:??) landed as
https://crrev.com/b2bc492b3a8709bd3398cf4ea18ddc1909286d2d
Cr-Commit-Position: refs/heads/master@{#332787}