Bug 1490115 - Add extra tests for comment-only inputs. r=glandium

--HG--
extra : rebase_source : 5ef9accd9c6d7501ff2e54a4a81b3360f0bbc040

modules/libpref/test/gtest/Parser.cpp

@@ -51,9 +51,7 @@ user_pref("string", "value");
   );
 
   // Totally empty input.
-  DEFAULT("",
+  DEFAULT("", "");
-    ""
-  );
 
   // Whitespace-only input.
   DEFAULT(R"(
@@ -62,6 +60,11 @@ user_pref("string", "value");
     ""
   );
 
+  // Comment-only inputs.
+  DEFAULT(R"(// blah)", "");
+  DEFAULT(R"(# blah)", "");
+  DEFAULT(R"(/* blah */)", "");
+
   //-------------------------------------------------------------------------
   // All the lexing errors. (To be pedantic, some of the integer literal
   // overflows are triggered in the parser, but put them all here so they're all