Update V8 DEPS.

Rolling v8/build: https://chromium.googlesource.com/chromium/src/build/+log/30bf2a3..d873086

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

Change-Id: I52816e40275461b012d218ae024261158db07dcb
Reviewed-on: https://chromium-review.googlesource.com/977262
Commit-Queue: v8 autoroll <v8-autoroll@chromium.org>
Reviewed-by: v8 autoroll <v8-autoroll@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52167}

DEPS

@@ -13,7 +13,7 @@ vars = {
 
 deps = {
   'v8/build':
-    Var('chromium_url') + '/chromium/src/build.git' + '@' + '30bf2a39946ba644e1ec693cd54c010362cb7a4d',
+    Var('chromium_url') + '/chromium/src/build.git' + '@' + 'd873086c6488e5265ff0c2273222fc449701d9b3',
   'v8/tools/gyp':
     Var('chromium_url') + '/external/gyp.git' + '@' + 'd61a9397e668fa9843c4aa7da9e79460fe590bfb',
   'v8/third_party/icu':