Update Windows build instructions

Update the Chromium build instructions to mention that VS 2019 is better
for debugging, and also specify how to request building Chromium with
VS 2019.

Change-Id: Icd41a375e41ba2a51472f4f459c1ecfecd225f46
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1654152
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Bruce Dawson <brucedawson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#668220}

docs/windows_build_instructions.md

@@ -24,6 +24,8 @@ Are you a Google employee? See
 ### Visual Studio
 
 Chromium requires Visual Studio 2017 (>=15.7.2) or 2019 (>=16.0.0) to build.
+Visual Studio can also be used to debug Chromium and Visual Studio 2019 is
+preferred for this as it handles Chromium's large debug information much better.
 The clang-cl compiler is used but Visual Studio's header files, libraries, and
 some tools are required. Visual Studio Community Edition should work if its
 license is appropriate for you. You must install the "Desktop development with
@@ -86,7 +88,9 @@ if your system PATH has a Python in it, you will be out of luck.
 
 Also, add a DEPOT_TOOLS_WIN_TOOLCHAIN system variable in the same way, and set
 it to 0. This tells depot_tools to use your locally installed version of Visual
-Studio (by default, depot_tools will try to use a google-internal version).
+Studio (by default, depot_tools will try to use a google-internal version). If
+you want to build with Visual Studio 2019 instead of Visual Studio 2017 (the
+default) then set the GYP_MSVS_VERSION environment variable to 2019.
 
 From a cmd.exe shell, run the command gclient (without arguments). On first
 run, gclient will install all the Windows-specific bits needed to work with