Update V8 to version 4.9.56.

Summary of changes available at:
https://chromium.googlesource.com/v8/v8/+log/6ed50d99..b787e6bc

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/
This only works with a Google account.

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

Review URL: https://codereview.chromium.org/1467413003

Cr-Commit-Position: refs/heads/master@{#361326}

DEPS

@@ -43,7 +43,7 @@ vars = {
   # Three lines of non-changing comments so that
   # the commit queue can handle CLs rolling V8
   # and whatever else without interference from each other.
-  'v8_revision': '6ed50d9918f6a109cdc8d5f5775194379d680edd',
+  'v8_revision': 'b787e6bc18649be65d0c62160154da59cd778f6b',
   # Three lines of non-changing comments so that
   # the commit queue can handle CLs rolling swarming_client
   # and whatever else without interference from each other.