Specify the SDK version when running vcvarsall.bat

Chromium requires the 10.0.14393.0 SDK to build. Previous versions don't
have all of the required declarations, and 10.0.15063.0 is buggy (it
includes a file that it doesn't ship with). So, we request 10.0.14393.0
exactly. This will fail if that SDK version is not installed.

The bug for the brokenness of 15063 is here:
https://developercommunity.visualstudio.com/content/problem/42961/15063-sdk-is-broken-bitsh-indirectly-references-no.html

Old SDKs (such as 14393) can be found here:
https://developer.microsoft.com/en-us/windows/downloads/sdk-archive

R=scottmg@chromium.org

Review-Url: https://codereview.chromium.org/2814843003
Cr-Commit-Position: refs/heads/master@{#463830}

build/toolchain/win/setup_toolchain.py

@@ -139,7 +139,10 @@ def _LoadToolchainEnv(cpu, sdk_dir):
         raise Exception('%s is missing - make sure VC++ tools are installed.' %
                         script_path)
       script_path = other_path
-    args = [script_path, 'amd64_x86' if cpu == 'x86' else 'amd64']
+    # Chromium requires the 10.0.14393.0 SDK. Previous versions don't have all
+    # of the required declarations, and 10.0.15063.0 is buggy.
+    args = [script_path, 'amd64_x86' if cpu == 'x86' else 'amd64',
+            '10.0.14393.0']
     variables = _LoadEnvFromBat(args)
   return _ExtractImportantEnvironment(variables)