Update V8 DEPS.

Rolling v8/tools/clang to 21c0f1864716dbe8d8d4061c18efbde7c49b6a6a

TBR=machenbach@chromium.org

Review URL: https://codereview.chromium.org/930743002

Cr-Commit-Position: refs/heads/master@{#26652}

DEPS

@@ -18,7 +18,7 @@ deps = {
   "v8/testing/gmock":
     Var("git_url") + "/external/googlemock.git" + "@" + "29763965ab52f24565299976b936d1265cb6a271",  # from svn revision 501
   "v8/tools/clang":
-    Var("git_url") + "/chromium/src/tools/clang.git" + "@" + "d5f568673280ba9180bbf0b913b571786c9f352a",
+    Var("git_url") + "/chromium/src/tools/clang.git" + "@" + "21c0f1864716dbe8d8d4061c18efbde7c49b6a6a",
 }
 
 deps_os = {