Update V8 to version 5.6.21.

Update V8 to version 5.6.21.

Summary of changes available at:
https://chromium.googlesource.com/v8/v8/+log/2b3ba461..c026428a

Please follow these instructions for assigning/CC'ing issues:
https://github.com/v8/v8/wiki/Triaging%20issues

Please close rolling in case of a roll revert:
https://v8-roll.appspot.com/
This only works with a Google account.

CQ_INCLUDE_TRYBOTS=master.tryserver.blink:linux_precise_blink_rel;master.tryserver.chromium.linux:linux_optional_gpu_tests_rel;master.tryserver.chromium.mac:mac_optional_gpu_tests_rel;master.tryserver.chromium.win:win_optional_gpu_tests_rel

TBR=hablich@chromium.org,machenbach@chromium.org,littledan@chromium.org,vogelheim@chromium.org

[v8-autoroll, 2016-10-11 10:18:31]

The CQ bit was checked by v8-autoroll@chromium.org

[commit-bot: I haz the power, 2016-10-11 10:18:37]

CQ is trying da patch. Follow status at
 
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.chromium.or...

[commit-bot: I haz the power, 2016-10-11 11:21:39]

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

[commit-bot: I haz the power, 2016-10-11 11:21:40]

Try jobs failed on following builders:
  win_chromium_rel_ng on master.tryserver.chromium.win (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.win/builders/win_chromium_rel_...)

[v8-autoroll, 2016-10-11 11:30:17]

The CQ bit was checked by v8-autoroll@chromium.org

[commit-bot: I haz the power, 2016-10-11 11:30:28]

CQ is trying da patch. Follow status at
 
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.chromium.or...

[commit-bot: I haz the power, 2016-10-11 12:39:35]

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

[commit-bot: I haz the power, 2016-10-11 12:39:36]

Try jobs failed on following builders:
  linux_chromium_rel_ng on master.tryserver.chromium.linux (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.linux/builders/linux_chromium_...)

[Michael Achenbach, 2016-10-11 12:43:51]

# Fatal error in ../../v8/src/crankshaft/hydrogen.cc, line 4010
  # Check failed: owner()->HasStackOverflow() || owner()->current_block() ==
__null || (owner()->environment()->length() == original_length_ + 1 &&
(owner()->environment()->frame_type() == JS_FUNCTION ||
owner()->environment()->frame_type() == TAIL_CALLER_FUNCTION)).