Static debug builds aren't really supported in Chromium (we don't have any builders that build them), and on some platforms (e.g., Windows), they flat out won't work.

There has been discussion off and on about whether we should support them, and whether we should default to shared (which tends to be a better dev experience since it builds faster and takes less disk space, but is farther away from the configuration we ship).

Historically, we couldn't easily change this in GYP because debug and release had to be either both static or both shared. However, in GN, there's no reason we can't have different defaults here.