Split toolchain update and install lookup to make generation faster

DEPS

Index: DEPS
diff --git a/DEPS b/DEPS
index b7a06f1efbf6a4516496924413c8ad52634c0366..5afb8c5432a840764930ff5ea991c192acffa2fc 100644
--- a/DEPS
+++ b/DEPS
@@ -637,6 +637,12 @@ hooks = [
     "action": ["python", "src/tools/clang/scripts/update.py", "--if-needed"],
   },
   {
+    # Update the Windows toolchain if necessary.
+    "name": "win_toolchain",
+    "pattern": ".",
+    "action": ["python", "src/build/vs_toolchain.py", "update"],
+  },
+  {
     # Update LASTCHANGE. This is also run by export_tarball.py in
     # src/tools/export_tarball - please keep them in sync.
     "name": "lastchange",