Force building with *spring* Creators Update SDK

The Fall Creators Update SDK has various bugs/incompatibilities that
cause errors when building Chromium. This will not affect Google
employees who use a packaged toolchain but good affect anybody who set
DEPOT_TOOLS_WIN_TOOLCHAIN=0 and is therefore using the system install
of VS 2017. This change forces vcvarsall.bat to use the 15063 SDK
instead of the latest-installed.

This should be a temporary measure that can be undone as we land
workarounds for the incompatibilities or get fixed SDK versions.

Bug: 773476
Change-Id: If5b89186d5053c878f5c771a489fe2082d9b2d68
Reviewed-on: https://chromium-review.googlesource.com/714366
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Reviewed-by: Daniel Bratell <bratell@opera.com>
Commit-Queue: Bruce Dawson <brucedawson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#508362}
diff --git a/build/toolchain/win/setup_toolchain.py b/build/toolchain/win/setup_toolchain.py
index 6308bbf..bb599d6 100644
--- a/build/toolchain/win/setup_toolchain.py
+++ b/build/toolchain/win/setup_toolchain.py
@@ -139,9 +139,11 @@
         raise Exception('%s is missing - make sure VC++ tools are installed.' %
                         script_path)
       script_path = other_path
-    # Chromium requires the 10.0.15063.468 SDK or higher - previous versions
-    # don't have all of the required declarations.
-    args = [script_path, 'amd64_x86' if cpu == 'x86' else 'amd64']
+    # Chromium requires the 10.0.15063.468 SDK - previous versions don't have
+    # all of the required declarations and 10.0.16299.0 has some
+    # incompatibilities (crbug.com/773476).
+    args = [script_path, 'amd64_x86' if cpu == 'x86' else 'amd64',
+            '10.0.15063.0']
     variables = _LoadEnvFromBat(args)
   return _ExtractImportantEnvironment(variables)