Auto-updating script for Windows toolchain

DEPS

Index: DEPS
diff --git a/DEPS b/DEPS
index ec52e306f9cb2143978e813017279a196b524c70..da5b7d22fb2d8fc087126a28e8d585edf9ea22b1 100644
--- a/DEPS
+++ b/DEPS
@@ -641,6 +641,14 @@ hooks = [
                "-s", "src/third_party/WebKit",
                "-o", "src/build/util/LASTCHANGE.blink"],
   },
+  # Windows toolchain setup.
+  {
+    "name": "toolchain_win",
+    "pattern": "third_party/win_toolchain/toolchain\\.sha1",
+    "action": [ "python",
+                "src/tools/win/toolchain/get_toolchain_if_necessary.py",
+    ],
+  },
   # Pull GN binaries. This needs to be before running GYP below.
   {
     "name": "gn_win",