Setting up Windows
You must set your Windows system locale to English, or else you may get build errors about "The file contains a character that cannot be represented in the current code page."
Setting up the environment for Visual Studio 2013
You must build with Visual Studio 2013 Update 4, no other versions are supported.
You must have Windows 7 x64 or later. x86 OSs are unsupported.
Step 2: Follow the appropriate path below:
Run set DEPOT_TOOLS_WIN_TOOLCHAIN=0, or set that variable in your global environment.
Note that Visual Studio Express is not supported and will not be able to build Chromium.
Run: download_from_google_storage --config and follow the authentication instructions. Note that you must authenticate with your @google.com credentials, not @chromium.org. [Note also that the instructions tell you to "hit enter when prompted for a project-id", but this crashes the script - enter "1" instead.]
Once you've done this, the toolchain will be installed automatically for you in Step 3, below (near the end of the step).
The toolchain will be in depot_tools\win_toolchain, and windbg can be found in depot_tools\win_toolchain\vs2013_files\win8sdk\Debuggers.
If you want the IDE for debugging and editing, you will need to install it separately, but this is optional and not needed to build Chromium.
If you're primarily going to be doing debug dev builds, you'll want to set GYP_DEFINES=component=shared_library, which uses a DLL build and incremental linking, which makes linking much faster in Debug.