Revert "Update V8 to version 7.6.206."

This reverts commit 3cf15f91.

Reason for revert:
https://ci.chromium.org/p/chromium/builders/ci/WebKit%20Linux%20Trusty%20Leak
08:51:48.392 3538 worker/2 external/wpt/wasm/webapi/body.any.html leaked
08:51:48.397 3431 [5197/7478] external/wpt/wasm/webapi/body.any.html failed unexpectedly (leak detected: ({"numberOfLiveDocuments":[1,2],"numberOfLiveNodes":[4,22],"numberOfLiveResourceFetchers":[1,2],"numberOfLiveResources":[0,3]}))
08:51:48.393 3538 worker/2 external/wpt/wasm/webapi/body.any.html failed:
08:51:48.393 3538 worker/2  leak detected: ({"numberOfLiveDocuments":[1,2],"numberOfLiveNodes":[4,22],"numberOfLiveResourceFetchers":[1,2],"numberOfLiveResources":[0,3]})

Since it appears WASM related and the V8 roll is in a pretty small set, this seems the most likely culprit.

https://chromium.googlesource.com/chromium/src.git/+log/2b9db0045e4d669a0615c40215de3cfb95e729ea..0daa25d5051667136a263f627aa0a61703f26dd4

Original change's description:
> Update V8 to version 7.6.206.
>
> Summary of changes available at:
> https://chromium.googlesource.com/v8/v8/+log/c9178292..92b8322b
>
> 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,v8-waterfall-sheriff@grotations.appspotmail.com
>
> Change-Id: Iac0d4b87103b6c56b70aad5f27f8a66405e62130
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1617067
> Reviewed-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@{#660835}

TBR=hablich@chromium.org,v8-ci-autoroll-builder@chops-service-accounts.iam.gserviceaccount.com,v8-waterfall-sheriff@grotations.appspotmail.com

Change-Id: I6ef39c00255ca2a86f9cddacc5ea4c5ec4122f8b
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1618163
Commit-Queue: Brian White <bcwhite@chromium.org>
Reviewed-by: Brian White <bcwhite@chromium.org>
Cr-Commit-Position: refs/heads/master@{#661010}

DEPS

@@ -137,7 +137,7 @@ 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': '92b8322b89a02a88c867f43e7b6770c779ff6d62',
+  'v8_revision': 'c9178292dbb05caf07f5e1cd985f90383fbba6b9',
   # 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.