This simple example works in pure html when launching within visual studio localhost:port server
Google is all Javascript, so the fact it's in a Master page shouldn't matter unless you have local path issues, which I don't see.
I would use Firefox's Error Console to see if there are any Javascript errors when the page loads.
EDIT
I thought it was some weird tag nesting issue, but it's actually the fact that you're using percentages for your dimensions. '100%' means use fill the enclosing containers space, but if the container is empty we don't get a size. If you set your map div to a fixed size, you will see your map.