Update V8 to version 7.4.8.1 (cherry-pick).

Summary of changes available at:
https://chromium.googlesource.com/v8/v8/+log/6bf2d799..6e89f8f5

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:mac_optional_gpu_tests_rel;luci.chromium.try:android_optional_gpu_tests_rel

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

Change-Id: I0658c329b9e3b453bd3e2571f9641d9129cd647e
Reviewed-on: https://chromium-review.googlesource.com/c/1440508Reviewed-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@{#626830}

DEPS

@@ -125,7 +125,7 @@ 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': '6bf2d7999b7d4193221f058a70962b181abf58d3',
+  'v8_revision': '6e89f8f552fe46cbf81a5bbb40066f881b2e7e65',
   # 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.