Update V8 DEPS.

Rolling v8/build: https://chromium.googlesource.com/chromium/src/build/+log/ff6a082..e840a06

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

Change-Id: Ie172612a3753a11ef85b4762b0e3f12891e040ad
Reviewed-on: https://chromium-review.googlesource.com/517727Reviewed-by: v8 autoroll <v8-autoroll@chromium.org>
Commit-Queue: v8 autoroll <v8-autoroll@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45580}

DEPS

@@ -8,7 +8,7 @@ vars = {
 
 deps = {
   "v8/build":
-    Var("chromium_url") + "/chromium/src/build.git" + "@" + "ff6a08293bc69c91b2243e4acec755e481a05d07",
+    Var("chromium_url") + "/chromium/src/build.git" + "@" + "e840a062ae6e44aea7d3abccf6f376c7d2dd0d00",
   "v8/tools/gyp":
     Var("chromium_url") + "/external/gyp.git" + "@" + "eb296f67da078ec01f5e3a9ea9cdc6d26d680161",
   "v8/third_party/icu":