Update V8 to version 4.6.56.

Update V8 to version 4.6.56.

Summary of changes available at:
https://chromium.googlesource.com/v8/v8/+log/0c1ebe48..b531751e

Please follow these instructions for assigning/CC'ing issues:
https://code.google.com/p/v8-wiki/wiki/TriagingIssues

Please close rolling in case of a roll revert:
https://v8-roll.appspot.com/

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

Review URL: https://codereview.chromium.org/1280023002

Cr-Commit-Position: refs/heads/master@{#342330}