tools/gn: Fix errors found by Facebook's flint tool.

tools/gn/test_with_scope.h

Index: tools/gn/test_with_scope.h
diff --git a/tools/gn/test_with_scope.h b/tools/gn/test_with_scope.h
index 449a504cfd6e91d1d7f71b0e2c851ed9bbff7548..26b082f9fca7358ef9fcdc34e29eaf91c056d81c 100644
--- a/tools/gn/test_with_scope.h
+++ b/tools/gn/test_with_scope.h
@@ -60,7 +60,7 @@ class TestWithScope {
 // then you can execute the ParseNode or whatever.
 class TestParseInput {
  public:
-  TestParseInput(const std::string& input);
+  explicit TestParseInput(const std::string& input);
   ~TestParseInput();
 
   // Indicates whether and what error occurred during tokenizing and parsing.