Update V8 DEPS.

Rolling v8/buildtools: https://chromium.googlesource.com/chromium/buildtools/+log/9a94713..cb12d6e

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

Review-Url: https://codereview.chromium.org/2638693002
Cr-Commit-Position: refs/heads/master@{#42353}

DEPS

@@ -16,7 +16,7 @@ deps = {
   "v8/third_party/instrumented_libraries":
     Var("chromium_url") + "/chromium/src/third_party/instrumented_libraries.git" + "@" + "5b6f777da671be977f56f0e8fc3469a3ccbb4474",
   "v8/buildtools":
-    Var("chromium_url") + "/chromium/buildtools.git" + "@" + "9a947138bc5851bce3e9ddb1c3420d12a8f27c25",
+    Var("chromium_url") + "/chromium/buildtools.git" + "@" + "cb12d6e8641f0c9b0fbbfa4bf17c55c6c0d3c38f",
   "v8/base/trace_event/common":
     Var("chromium_url") + "/chromium/src/base/trace_event/common.git" + "@" + "06294c8a4a6f744ef284cd63cfe54dbf61eea290",
   "v8/third_party/jinja2":