Document CDB dependency in Windows build for external contributors.

docs/windows_build_instructions.md

 # Windows Build Instructions
 
 ## Common checkout instructions
 
 This page covers Windows-specific setup and configuration. The
 [general checkout
 instructions](http://dev.chromium.org/developers/how-tos/get-the-code) cover
 installing depot tools and checking out the code via git.
 
 ## Setting up Windows
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 should work if its license is appropriate for you. Use the Custom Install option
 and select:
 
 - Visual C++, which will select three sub-categories including MFC
 - Universal Windows Apps Development Tools > Tools
 - Universal Windows Apps Development Tools > Windows 10 SDK (10.0.10586)
 
 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.
 
 Compilation is done through ninja, **not** Visual Studio.
 
 ### Google employees
 
 Run: `download_from_google_storage --config` and follow the
 authentication instructions. **Note that you must authenticate with your
 @google.com credentials**, not @chromium.org. Enter "0" if asked for a
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 3.  Store the build tree on a fast disk (preferably SSD).
 4.  If you are primarily going to be doing debug development builds, you
     should use the component build. Set the [build
     arg](https://www.chromium.org/developers/gn-build-configuration)
     `is_component_build = true`.
     This will generate many DLLs and enable incremental linking, which makes
     linking **much** faster in Debug.
 
 Still, expect build times of 30 minutes to 2 hours when everything has to
 be recompiled.