I am seeing the same problem here, and have a solution that works in my test situation.
I am using a "va_stdafx.h" file, which is explained here:
https://support.wholetomato.com/default.asp?W302
and using the file content:
#define RTC_GUARDED_BY(acm_mutex_)
to tell VA to ignore this macro after the variable.
This file should be placed in the same directory as your .SLN or .VCXPROJ file. Are you working with a .SLN file, or are you working with a CMake build, or something else?
In my test I have simply cloned the webrtc repositiry with git, and then placed the "va_stdafx.h" file in the root folder, and told Visual Studio 2019 to open this root folder directly. VA is then picking up and using this file. Since VA looks for the file directly there is no need to try adding this file to your solution, but if you are building with CMake, or something else that picks up all .h files in this root directory this might cause problems.
This file is used to help VA's parser with difficult code, and can be used to work around odd effects. After modifying this file you need to rebuild the VA symbol database for the changes to take effect:
VA Options -> Performance -> General -> Rebuild symbol databases