Update the V8 version for Chromium branch 81.

This is a roll-back to 8.1.307. This is the most stable candidate out of the latest Canaries.

If this commit results in failures in Blink please contact the Blink sheriff. They may need to make changes to Blink because of the roll-back.

When in doubt please try to contact the committer and reviewers of this CL before reverting it.

TBR=machenbach@chromium.org

Change-Id: I85ae6732d8d5869ef68d39aa9481ae6a974d6963
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2030409Reviewed-by: Michael Hablich <hablich@chromium.org>
Commit-Queue: Michael Hablich <hablich@chromium.org>
Auto-Submit: Michael Hablich <hablich@chromium.org>
Cr-Commit-Position: refs/heads/master@{#736828}

DEPS

@@ -181,7 +181,7 @@ 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': '62aeb04515990c92d82af9f4843d4cdb4085a9a5',
+  'v8_revision': 'a4dcd39d521d14c4b1cac020812e44ee04a7f244',
   # 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.