BackgroundSync sync events need to be ExtendableEvents, content side cleanup

BackgroundSync sync events need to be ExtendableEvents, cleanup

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

[1/4] https://codereview.chromium.org/1162243003/, 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] This patch, cleanup on content side

Specific changes in this patch:
- Remove obsolete function from content worker context client
- Overloaded function made obsolete by landing blink and content patches

BUG=486890
Committed: https://crrev.com/124f100e5173e39d671b759d8d897e55bf209ba1
Cr-Commit-Position: refs/heads/master@{#334946}

[jkarlin, 2015-06-17 17:00:41]

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

[jkarlin, 2015-06-17 17:00:41]

lgtm!

[chasej, 2015-06-17 21:05:14]

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

[chasej, 2015-06-17 21:05:39]

michaeln, please take a look.

[michaeln, 2015-06-17 22:28:17]

lgtm 2 (hooray deletes)

[chasej, 2015-06-17 22:42:37]

The CQ bit was checked by chasej@chromium.org

[chasej, 2015-06-17 22:42:38]

The patchset sent to the CQ was uploaded after l-g-t-m from jkarlin@chromium.org
Link to the patchset: https://codereview.chromium.org/1146063005/#ps20001
(title: "Rebase")

[commit-bot: I haz the power, 2015-06-17 22:43:31]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1146063005/20001

[commit-bot: I haz the power, 2015-06-17 22:49:11]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-06-17 22:51:13]

Patchset 2 (id:??) landed as
https://crrev.com/124f100e5173e39d671b759d8d897e55bf209ba1
Cr-Commit-Position: refs/heads/master@{#334946}