Replace ScriptStreamerTask with blink::Task and threadSafeBind()

Replace ScriptStreamerTask with blink::Task and threadSafeBind()

This is to
- make cross-thread task posting explicit, and
- reduce cross-thread data passing outside threadSafeBind().

BUG=478194
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=196047

[hiroshige, 2015-05-28 05:28:18]

The CQ bit was checked by hiroshige@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2015-05-28 05:28:31]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1143083005/1

[commit-bot: I haz the power, 2015-05-28 06:53:20]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-05-28 06:53:21]

Dry run: This issue passed the CQ dry run.

[haraken, 2015-05-28 06:59:17]

Would you add a more detailed description?

[hiroshige, 2015-05-28 08:14:16]

hiroshige@chromium.org changed reviewers:
+ haraken@chromium.org

[hiroshige, 2015-05-28 08:15:10]

On 2015/05/28 06:59:17, haraken wrote:
> Would you add a more detailed description?

Added. (also fixed a bug in Patch Set 2).
PTAL.

[hiroshige, 2015-05-28 08:15:19]

The CQ bit was checked by hiroshige@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2015-05-28 08:15:35]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1143083005/20001

[haraken, 2015-05-28 08:25:08]

LGTM

[hiroshige, 2015-05-28 08:30:38]

The CQ bit was unchecked by hiroshige@chromium.org

[hiroshige, 2015-05-28 08:33:20]

The CQ bit was checked by hiroshige@chromium.org

[commit-bot: I haz the power, 2015-05-28 08:33:42]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1143083005/20001

[commit-bot: I haz the power, 2015-05-28 09:38:52]

Committed patchset #2 (id:20001) as
https://src.chromium.org/viewvc/blink?view=rev&revision=196047