Update Windows build instruction for 14393 SDK

The Windows build instructions mention that the 14393 SDK is needed but
now that 15063 has shipped the SDK is harder to find, leading some
developers to do problematic hacks. This adds a link to the archival
installer.

R=scottmg@chromium.org

Review-Url: https://codereview.chromium.org/2846643002
Cr-Commit-Position: refs/heads/master@{#467512}

docs/windows_build_instructions.md

@@ -34,12 +34,14 @@ and select:
 - Universal Windows Apps Development Tools > Tools (1.4.1) and Windows 10 SDK
   (10.0.14393)
 
-You must have the 14393 SDK installed or else you will hit compile errors such
-as undefined or redefined macros.
-
-Install the Windows SDK 10, and choose Debugging Tools For Windows when you
-install this in order to get windbg and cdb. The latter is required for the
-build to succeed as some tests use it for symbolizing crash dumps.
+You must have the 14393 Windows SDK installed - the 15063 SDK has errors and
+cannot be used to compile Chrome. It is okay to have multiple SDK versions
+installed as long as 14393 is one of them. The installer can be found in the
+[Windows SDK archive](https://developer.microsoft.com/en-us/windows/downloads/sdk-archive).
+
+When installing the 14393 Windows SDK choose Debugging Tools For Windows in
+order to get windbg and cdb. The latter is required for the build to succeed as
+some tests use it for symbolizing crash dumps.
 
 ## Install `depot_tools`