Update the mojom lexer to emit comment tokens.

mojom/mojom_parser/lexer/tokens.go

Index: mojom/mojom_parser/lexer/tokens.go
diff --git a/mojom/mojom_parser/lexer/tokens.go b/mojom/mojom_parser/lexer/tokens.go
index 660a84e574910eeffc5eb2192385520510cbb79a..0083ba2504bcae69106b0b56751e91fce2c77ea6 100644
--- a/mojom/mojom_parser/lexer/tokens.go
+++ b/mojom/mojom_parser/lexer/tokens.go
@@ -67,6 +67,10 @@ const (
 	FloatConst
 	Ordinal
 	StringLiteral
+
+	// Comments
+	SingleLineComment
+	MultiLineComment
 )
 
 // This method is used to generate user-facing strings in compilation error
@@ -162,6 +166,11 @@ func (tokenKind TokenKind) String() string {
 	case StringLiteral:
 		return "a string literal"
 
+	case SingleLineComment:
+		return "single line comment"
+	case MultiLineComment:
+		return "multi line comment"
+
 	default:
 		// Note(rudominer) It is important to use %d below so as to avoid
 		// re-invoking this method and causing an infinite recursion.