Implement blink::Platform::yieldCurrentThread()

Implement blink::Platform::yieldCurrentThread()

Since we use an embedder provided thread everywhere, we should also let
the embedder do the yielding.

BUG=301514
R=jamesr@chromium.org

Committed: https://crrev.com/82b9096939b7e60095fc822c31975b25ee22e37b
Cr-Commit-Position: refs/heads/master@{#296432}

[jamesr, 2014-09-24 15:24:05]

lgtm

[jochen (gone - plz use gerrit), 2014-09-24 15:25:17]

The CQ bit was checked by jochen@chromium.org

[commit-bot: I haz the power, 2014-09-24 15:25:51]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/596243002/1

[commit-bot: I haz the power, 2014-09-24 16:15:41]

Committed patchset #1 (id:1) as f99339727393b302ca61f972fafc0b8158eaf6b8

[commit-bot: I haz the power, 2014-09-24 16:16:24]

Patchset 1 (id:??) landed as
https://crrev.com/82b9096939b7e60095fc822c31975b25ee22e37b
Cr-Commit-Position: refs/heads/master@{#296432}