Run microtasks at the end of each worker task.

Run microtasks at the end of each worker task.

This CL adds a TaskObserver that run Microtasks at the end of each Task
to WorkerThread. This CL also fixes a Promise resolution timing issue
caused by V8RecursionScope in the ScriptPromiseResolver implementation.
Because the microtask running timing has changed, We fix affected tests.

BUG=374772
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=179922

[yhirano, 2014-08-08 05:57:40]

jrummel@, This CL change the timing when promise handlers are called.
Previously, they were called synchronously when ScriptPromiseResolver is
resolved or rejected. This CL makes it asynchronous.
Hence I changed the tests. They counted a ScriptPromiseResolver in
numActiveDOMObjectsCreated, but this CL makes the handlers called after the
ScriptPromiseResolver is destructed.

[adamk, 2014-08-08 16:51:51]

lgtm!

[jrummell, 2014-08-08 16:58:03]

I like the change.

LayoutTests/media LGTM.

[yhirano, 2014-08-11 03:27:00]

The CQ bit was checked by yhirano@chromium.org

[commit-bot: I haz the power, 2014-08-11 03:27:42]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/yhirano@chromium.org/441063003/60001

[commit-bot: I haz the power, 2014-08-11 04:29:12]

FYI, CQ is re-trying this CL (attempt #1).
The failing builders are:
  win_blink_rel on tryserver.blink
(http://build.chromium.org/p/tryserver.blink/builders/win_blink_rel/builds/21951)

[commit-bot: I haz the power, 2014-08-11 05:34:00]

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

[commit-bot: I haz the power, 2014-08-11 05:34:02]

Try jobs failed on following builders:
  win_blink_rel on tryserver.blink
(http://build.chromium.org/p/tryserver.blink/builders/win_blink_rel/builds/21957)

[yhirano, 2014-08-11 07:36:14]

The CQ bit was checked by yhirano@chromium.org

[commit-bot: I haz the power, 2014-08-11 07:36:41]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/yhirano@chromium.org/441063003/100001

[commit-bot: I haz the power, 2014-08-11 08:34:10]

Change committed as 179922