Update V8 to version 6.9.155.

Summary of changes available at:
https://chromium.googlesource.com/v8/v8/+log/4b7910b8..7124db04

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=master.tryserver.blink:linux_trusty_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,machenbach@chromium.org,kozyatinskiy@chromium.org,sergiyb@chromium.org

Change-Id: If22570fceb3bfec7f7fb4d8692476a7d58907d1b
Reviewed-on: https://chromium-review.googlesource.com/1100973Reviewed-by: V8 Autoroller <v8-ci-autoroll-builder@chops-service-accounts.iam.gserviceaccount.com>
Commit-Queue: V8 Autoroller <v8-ci-autoroll-builder@chops-service-accounts.iam.gserviceaccount.com>
Cr-Commit-Position: refs/heads/master@{#567320}

DEPS

@@ -109,7 +109,7 @@ 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': '4b7910b87e6819ed58525efba297a2285dfe02f2',
+  'v8_revision': '7124db041bab998f849c91064734513b2014952b',
   # 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.