Close your chrome and find a file called "Local State".
For Windows Vista/7/8 it is usually located in "%LOCALAPPDATA%\Google\Chrome\user data\Local State" (include the "")
For Linux, maybe Mac OS X and others, too it is located in "~/.config/google-chrome/Local State" (include the "")
Open the file for editing and search for "protocol_handler".
Make sure you are NOT using notepad. Real men use ViM.. or notepad++.. or textpad.. anything really, just not notepad.exe ;-)
You should see something along the lines of: "protocol_handler": {
"excluded_schemes": {
"afp": true,
"data": true,
"disk": true,
"disks": true,
"file": true,
"gogdownloader": false,
"hcp": true,
"javascript": true,
"kindle": true,
"mailto": false,
"ms-help": true,
"news": false,
"nntp": true,
"shell": true,
"snews": false,
"steam": false,
"vbscript": true,
"view-source": true,
"vnd": {
"ms": {
"radio": true
}
}
}
},
Remove those lines with protocol names that you want to reset back to defaults and then save the file.
Start Chrome and head on over to the site that wants to use the protocol handler in question, and presto, it should ask you what to do about it (like you never decided to ignore site handler requests for this protocol in the first place).