Clarify that cdb is required to be installed

docs/windows_build_instructions.md

 # Checking out and Building Chromium for Windows
 
 There are instructions for other platforms linked from the 
 [get the code](get_the_code.md) page.
 
 ## Instructions for Google Employees
 
 Are you a Google employee? See
 [go/building-chrome-win](https://goto.google.com/building-chrome-win) instead.
 
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 and select:
 
 - Visual C++, which will select three sub-categories including MFC
 - 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.
+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.
 
 ## Install `depot_tools`
 
 Download the [depot_tools bundle](https://storage.googleapis.com/chrome-infra/depot_tools.zip)
 and extract it somewhere.
 
 *** note
 **Warning:** **DO NOT** use drag-n-drop or copy-n-paste extract from Explorer,
 this will not extract the hidden “.git” folder which is necessary for
 depot_tools to autoupdate itself. You can use “Extract all…” from the 
@@ skipping 174 matching lines @@
 $ gclient sync
 ```
 
 The first command updates the primary Chromium source repository and rebases
 any of your local branches on top of tip-of-tree (aka the Git branch `origin/master`).
 If you don't want to use this script, you can also just use `git pull` or 
 other common Git commands to update the repo.
 
 The second command syncs the subrepositories to the appropriate versions and
 re-runs the hooks as needed.