Document CDB dependency in Windows build for external contributors.

The Windows 10 SDK installed by Visual Studio 2015 Community Edition
does not include Debugging Tools for Windows. The build process requires
CDB. According to the MSDN page on Debugging Tools, they are also
bundled with the Windows Driver Kit, which is freely available. So, we
can point our external contributors to the WDK to get CDB.

BUG=639212

Review-Url: https://codereview.chromium.org/2263513002
Cr-Commit-Position: refs/heads/master@{#413506}

Author: pwnall

docs/windows_build_instructions.md

@@ -39,6 +39,9 @@ and select:
 You must have the 10586 SDK installed or else you will hit compile errors such
 as redefined macros.
 
+Install Windows Driver Kit (WDK) 10, or use some other method to get the
+Debugging Tools for Windows.
+
 Run `set DEPOT_TOOLS_WIN_TOOLCHAIN=0`, or set that variable in your
 global environment.