Update V8 to version 8.9.121.

Summary of changes available at:
https://chromium.googlesource.com/v8/v8/+log/53a11922..4670e2c2

Please follow these instructions for assigning/CC'ing issues:
https://v8.dev/docs/triage-issues

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
CQ_INCLUDE_TRYBOTS=luci.chromium.try:linux_optional_gpu_tests_rel
CQ_INCLUDE_TRYBOTS=luci.chromium.try:mac_optional_gpu_tests_rel
CQ_INCLUDE_TRYBOTS=luci.chromium.try:win_optional_gpu_tests_rel
CQ_INCLUDE_TRYBOTS=luci.chromium.try:android_optional_gpu_tests_rel

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

Change-Id: I30ea9a65edbffd602b8ed24b05c0c1385779165c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2578865Reviewed-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@{#834744}

DEPS

@@ -203,7 +203,7 @@ 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': '53a11922d89285a3f15744d59e0cfcff2dd33f91',
+  'v8_revision': '4670e2c2930e89462938e85ba131caeb7c0d0419',
   # 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.