Update V8 to version 7.4.256.

Summary of changes available at:
https://chromium.googlesource.com/v8/v8/+log/96fc7ac1..16c76cb9

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=luci.chromium.try:linux-blink-rel;luci.chromium.try:linux_optional_gpu_tests_rel;luci.chromium.try:mac_optional_gpu_tests_rel;luci.chromium.try:win_optional_gpu_tests_rel;luci.chromium.try:android_optional_gpu_tests_rel

TBR=hablich@chromium.org,v8-waterfall-sheriff@grotations.appspotmail.com

Change-Id: Ie7c9ac6c88ad2abc84a83c97f8666563c79bc8b1
Reviewed-on: https://chromium-review.googlesource.com/c/1496247Reviewed-by: v8-ci-autoroll-builder <v8-ci-autoroll-builder@chops-service-accounts.iam.gserviceaccount.com>
Commit-Queue: v8-ci-autoroll-builder <v8-ci-autoroll-builder@chops-service-accounts.iam.gserviceaccount.com>
Cr-Commit-Position: refs/heads/master@{#636667}

DEPS

@@ -131,7 +131,7 @@ 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': '96fc7ac109d13d86d67572349a5eec7f1540abdd',
+  'v8_revision': '16c76cb9342d8f95ccf4faddd1418a6a2f594b3e',
   # 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.