Add README.md to tools/win/DebugVisualizers

NOTRY=True

Change-Id: Ib088a06a2f6e85ac5cb23c6c8deb0b6b9eeae7e3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2470038Reviewed-by: Bruce Dawson <brucedawson@chromium.org>
Reviewed-by: Yoshifumi Inoue <yosin@chromium.org>
Commit-Queue: Koji Ishii <kojii@chromium.org>
Cr-Commit-Position: refs/heads/master@{#817399}

tools/win/DebugVisualizers/README.md

+This directory contains "natvis" (Native Visualization) files,
+that provide custom views in the Visual Studio debugger.
+The official Natvis documentation is located at
+[Create custom views of C++ objects in the debugger using the Natvis
+framework](https://docs.microsoft.com/en-us/visualstudio/debugger/create-custom-views-of-native-objects).
+
+We have not found a good way to automate testing these files.
+The following process is recommended before updating these files.
+
+## Preparations
+
+These files are embedded in `.lib`, `.dll`, or `.exe` so that the debugger can
+use them without referring to the original `.natvis` files.
+
+When you edit them, you can run a full build to update all of them, but
+following steps will help to avoid the full build, and to benefit from Visual
+Studio's dynamic reloading feature.
+
+1. Comment out the `.natvis` you want to edit in `BUILD.gn`. You can find them
+by [Chromium Code Search](https://source.chromium.org/search?q=.natvis%20file:BUILD.gn&ss=chromium).
+2. Create a directory "`Visual Studio 2019\Visualizers`" in your Documents
+folder if it does not exist.
+3. Copy the `.natvis` files you want to edit to the directory.
+
+With these steps done, if you save the `.natvis` files in your Documents folder
+while you are debugging, Visual Studio will automatically reload them, run
+diagnostic and show messages to Output window if any, and update
+Auto/Local/Watch windows without stopping the current debug session.
+
+## Diagnostic
+
+1. Go to Tools - Options, Debugging, Output Window.
+Select "Warning" for "Natvis diagnostic messages (C++ only)".
+
+With this set, Visual Studio shows diagnostic warning and error messages to the
+Output window when it reads `.natvis` files.
+
+## Testing
+
+It is recommended to do at least following tests before updating these files.
+
+1. Enable the diagnostic warnings above.
+2. Start the debugger and break somewhere where you can watch variables of the
+types you're interested.
+3. Check the Output window and verify that there are no errors or warnings for
+the files you edit.
+4. Check the types you modified are displayed correctly in Auto/Local/Watch
+windows.
+5. If the type has `Condition` or `IncludeView` attributes, they are like
+"if"-statements. Please make sure that your test function exercises all
+conditions and views.
+
+If there is not a good test that uses the variables of interest then a test
+should be created. It can be a test that does nothing except declare the
+variables of interest (using base::debug::Alias to prevent them from being
+optimized away). Whether it is a new or existing test it should be listed in the
+commit message to aid reviewers in doing an actual verification that the changes
+work. As an example, consider `TEST(ValuesTest, TakeList)` which is very helpful
+for testing `base::Value visualizers`.
\ No newline at end of file