Update the V8 version for Chromium branch 71.

This is a roll-back to V8 7.1.302. 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: I6c3ffe99cbd687bbef7b381ab4fdf38af2b58505
Reviewed-on: https://chromium-review.googlesource.com/c/1275952Reviewed-by: Michael Hablich <hablich@chromium.org>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Michael Hablich <hablich@chromium.org>
Cr-Commit-Position: refs/heads/master@{#598739}

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': '80d8889e4e4eead687e2aabe7dc60c23c17bd3a1',
+  'v8_revision': 'f70aaa8ab2e8815505a6145c745e50d8328cd28c',
   # 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.