Make all FATAL-related output appear as 'F' log output.

Previously most of the spew belonging to a FATAL would be 'E' because
Runtime::Abort() used LOG(ERROR) to avoid recursion.

Change-Id: If8f62586c302c239e827c5235ed58ee8fe6e8f5e
diff --git a/src/log_severity.h b/src/log_severity.h
index 5ecfe8d..126019b 100644
--- a/src/log_severity.h
+++ b/src/log_severity.h
@@ -20,5 +20,6 @@
 typedef int LogSeverity;
 
 const int VERBOSE = 0, DEBUG = 1, INFO = 2, WARNING = 3, ERROR = 4, FATAL = 5;
+const int INTERNAL_FATAL = 6; // For Runtime::Abort.
 
 #endif  // BASE_LOG_SEVERITY_H_