Building chromium takes multiple rebaseline operations.

Full logs and details are in https://docs.google.com/document/d/1GZ2-OqSsJyMCzmoQQ3_wu9s96TqXNg5xDdN04sofh0I

Machine is running Win10 Pro
From a vs2015 command prompt, with depot_tools on the path
DEPOT_TOOLS_WIN_TOOLCHAIN=0 [env var]
WindowsSDKVersion=10.0.15063.0\ [env var]
VS2015 and VS2017 installed

>gn args win.debug
[accept the empty args.gn file]

>ninja -C win.debug
[lots of output, including three requests to rebaseline]

After completing the rebaseline, the code did compile. [There was a separate bug with precompiled headers, which is in issue 763431]