Update V8 DEPS.

Rolling v8/build: https://chromium.googlesource.com/chromium/src/build/+log/fb3b9e0..81c45bb

TBR=machenbach@chromium.org,tmrts@chromium.org

Change-Id: I1374c82fa2994c76e80bfd8cf6074de63597a2e5
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2178131Reviewed-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@{#67524}

DEPS

@@ -72,7 +72,7 @@ vars = {
 
 deps = {
   'v8/build':
-    Var('chromium_url') + '/chromium/src/build.git' + '@' + 'fb3b9e03fc646403d6168708275d58f9e4330ecf',
+    Var('chromium_url') + '/chromium/src/build.git' + '@' + '81c45bb2bdf4963f2e9dd5eaf55fdf54259f6389',
   'v8/third_party/depot_tools':
     Var('chromium_url') + '/chromium/tools/depot_tools.git' + '@' + '082a11aa492205ba8470b2d41dce40a9979a3509',
   'v8/third_party/icu':