Update V8 to version 6.8.123.

Summary of changes available at:
https://chromium.googlesource.com/v8/v8/+log/4973c626..fa6b64e1

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: I9b238238ab208158bc60a469549f81b03fe675ff
Reviewed-on: https://chromium-review.googlesource.com/1038543Reviewed-by: v8 autoroll <v8-autoroll@chromium.org>
Commit-Queue: v8 autoroll <v8-autoroll@chromium.org>
Cr-Commit-Position: refs/heads/master@{#555239}

DEPS

@@ -100,7 +100,7 @@ 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': '4973c626b9be60e315ca23c9eb408366b9bf18fb',
+  'v8_revision': 'fa6b64e1f31666db1ec1f3a0339be62acf2e24f4',
   # 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.