win: add comment to help diagnose disallowed windows.h include

Add a comment to windows_h_disallowed.h to give a quick way to find
the include path for a disallowed include of Windows.h.

Also add an include of windows_h_disallowed.h to
content/browser/devtools/protocol/webauthn_handler.cc to give a more
useful message than obscure errors about GetUserNameW if Windows.h is
included.

Bug: 40555612
Change-Id: I4dd948dc9ece10e695fce405ace87c0425724935
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5988431
Reviewed-by: Jesse McKenna <jessemckenna@google.com>
Commit-Queue: David Bienvenu <davidbienvenu@chromium.org>
Reviewed-by: Danil Somsikov <dsv@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1379722}

base/win/windows_h_disallowed.h

@@ -13,6 +13,22 @@
 // #include "base/win/windows_h_disallowed.h"
 // #endif  // defined(_WINDOWS_)
 //
+// If it is not obvious why Windows.h is getting included, a quick way to
+// determine the #include path is to edit your local Windows.h (e.g.,
+// third_party\depot_tools\win_toolchain\vs_files\698eb5635a\Windows Kits\10\
+// Include\10.0.26100.0\um\Windows.h) to add
+//
+// #error Windows.h included
+//
+// at the top and recompile the file(s) that are unexpectedly including
+// Windows.h, e.g.,
+//
+// autoninja -C out/Default
+// obj\chrome\browser\devtools\devtools\devtools_window.obj
+//
+// That will generate a compile error with the #include path.
+// Then, undo the change to Windows.h.
+//
 // See https://crbug.com/796644 for more historical context.
 
 #ifndef BASE_WIN_WINDOWS_H_DISALLOWED_H_

content/browser/devtools/protocol/webauthn_handler.cc

@@ -29,6 +29,11 @@
 #include "device/fido/virtual_fido_device.h"
 #include "device/fido/virtual_u2f_device.h"
 
+// This should be after all other #includes.
+#if defined(_WINDOWS_)  // Detect whether windows.h was included.
+#include "base/win/windows_h_disallowed.h"
+#endif  // defined(_WINDOWS_)
+
 namespace content::protocol {
 
 namespace {