[renderer] Add support for postIdleTaskAfterWakeup to WebSchedulerImpl.

[renderer] Add support for postIdleTaskAfterWakeup to WebSchedulerImpl.

Adds support for postIdleTaskAfterWakeup to WebSchedulerImpl for use in
Blink.

Blink side of patch: https://codereview.chromium.org/940433005/

This will be required for implementation of long idle task - design doc:
https://docs.google.com/a/chromium.org/document/d/1yBlUdYW8VTIfB-DqhvQqUeP0kf-Ap1W4cao2yQq58Do/edit

BUG=455713
Committed: https://crrev.com/80b19ce509cc7ecad9338ec68342e3c7e55acf54
Cr-Commit-Position: refs/heads/master@{#317814}

[rmcilroy, 2015-02-18 20:45:48]

rmcilroy@chromium.org changed reviewers:
+ mkwst@chromium.org

[rmcilroy, 2015-02-18 20:45:49]

Mike, could you please take a look?  Thanks.

[Mike West, 2015-02-19 09:34:17]

LGTM, but you'll need a content/renderer OWNER.

[rmcilroy, 2015-02-24 15:44:11]

The CQ bit was checked by rmcilroy@chromium.org

[rmcilroy, 2015-02-24 15:44:11]

The patchset sent to the CQ was uploaded after l-g-t-m from mkwst@chromium.org
Link to the patchset: https://codereview.chromium.org/936853002/#ps20001 (title:
"Rebase")

[commit-bot: I haz the power, 2015-02-24 15:50:34]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/936853002/20001

[commit-bot: I haz the power, 2015-02-24 17:30:14]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-02-24 17:30:42]

Patchset 2 (id:??) landed as
https://crrev.com/80b19ce509cc7ecad9338ec68342e3c7e55acf54
Cr-Commit-Position: refs/heads/master@{#317814}