Remove old TODO and commented-out DCHECK in Isolate::RunMicrotasks

Remove old TODO and commented-out DCHECK in Isolate::RunMicrotasks

Per discussion on the bug, the DCHECK'd case actually occurs in normal
operation, outside of V8's control.

BUG=chromium:388244
LOG=n

Committed: https://crrev.com/6f4c0614d56087ef68345af967d8585166262b66
Cr-Commit-Position: refs/heads/master@{#29877}

[adamk, 2015-07-25 00:03:05]

adamk@chromium.org changed reviewers:
+ jochen@chromium.org

[jochen (gone - plz use gerrit), 2015-07-27 08:52:08]

lgtm

[adamk, 2015-07-27 15:35:42]

The CQ bit was checked by adamk@chromium.org

[commit-bot: I haz the power, 2015-07-27 15:35:49]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1255043002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1255043002/1

[commit-bot: I haz the power, 2015-07-27 15:38:45]

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

[commit-bot: I haz the power, 2015-07-27 15:38:46]

Try jobs failed on following builders:
  v8_presubmit on tryserver.v8 (JOB_FAILED,
http://build.chromium.org/p/tryserver.v8/builders/v8_presubmit/builds/4483)

[adamk, 2015-07-27 15:41:35]

The CQ bit was checked by adamk@chromium.org

[commit-bot: I haz the power, 2015-07-27 15:41:48]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1255043002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1255043002/1

[commit-bot: I haz the power, 2015-07-27 15:58:32]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-07-27 15:58:50]

Patchset 1 (id:??) landed as
https://crrev.com/6f4c0614d56087ef68345af967d8585166262b66
Cr-Commit-Position: refs/heads/master@{#29877}