Update V8 to version 4.6.57.

Summary of changes available at:
https://chromium.googlesource.com/v8/v8/+log/b531751e..38a68d3b

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/1282563002

Cr-Commit-Position: refs/heads/master@{#342401}

DEPS

@@ -38,7 +38,7 @@ 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': 'b531751e7bfc862b73a0bc67a8431047cbf4a298',
+  'v8_revision': '38a68d3b7596a2eca1f36f8bb8e3e124efbabd3c',
   # 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.