Update V8 DEPS.

Rolling v8/build: https://chromium.googlesource.com/chromium/src/build/+log/b8ed7ac..bb21bc0

TBR=machenbach@chromium.org,vogelheim@chromium.org,hablich@chromium.org

Change-Id: Ice2dcc25e1903c22c6e802048798ac02569808c5
Reviewed-on: https://chromium-review.googlesource.com/530765Reviewed-by: v8 autoroll <v8-autoroll@chromium.org>
Commit-Queue: v8 autoroll <v8-autoroll@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45834}

DEPS

@@ -8,7 +8,7 @@ vars = {
 
 deps = {
   "v8/build":
-    Var("chromium_url") + "/chromium/src/build.git" + "@" + "b8ed7ac7156797a2e32d4db7c253413bc8d8df42",
+    Var("chromium_url") + "/chromium/src/build.git" + "@" + "bb21bc06b90c55eb4465f1d6d944b9ceeb64e886",
   "v8/tools/gyp":
     Var("chromium_url") + "/external/gyp.git" + "@" + "eb296f67da078ec01f5e3a9ea9cdc6d26d680161",
   "v8/third_party/icu":