BackgroundSync sync events need to be ExtendableEvents, blink side cleanup

BackgroundSync sync events need to be ExtendableEvents, blink side cleanup

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

[1] https://codereview.chromium.org/1162243003/, content side fix
[2] https://codereview.chromium.org/1148293011/, blink side fix
[3] https://codereview.chromium.org/1160253004/, content browser tests
[4] https://codereview.chromium.org/1146063005/, cleanup on content side
[5] This patch, blink side cleanup

Specific changes in this patch:
- Remove obsolete function from public interface for worker context client
- Function only existed to prevent compile errors while landing the series of blink and content patches

BUG=486890
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=197344

[chasej, 2015-06-18 04:58:19]

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

[chasej, 2015-06-18 04:59:06]

tkent, please take a look.

[tkent, 2015-06-18 04:59:48]

lgtm

[chasej, 2015-06-18 11:40:46]

The CQ bit was checked by chasej@chromium.org

[commit-bot: I haz the power, 2015-06-18 11:41:48]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1191083002/1

[commit-bot: I haz the power, 2015-06-18 11:45:58]

Committed patchset #1 (id:1) as
https://src.chromium.org/viewvc/blink?view=rev&revision=197344