gn: make gn gen preserve import statements passed in --args

tools/gn/command_format_unittest.cc

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "base/files/file_util.h"
 #include "base/path_service.h"
 #include "base/strings/string_util.h"
 #include "testing/gtest/include/gtest/gtest.h"
+#include "tools/gn/command_format.h"
 #include "tools/gn/commands.h"
 #include "tools/gn/setup.h"
 
-namespace commands {
-bool FormatFileToString(Setup* setup,
-                        const SourceFile& file,
-                        bool dump_tree,
-                        std::string* output);
-}  // namespace commands
-
 #define FORMAT_TEST(n)                                                      \
   TEST(Format, n) {                                                         \
     ::Setup setup;                                                          \
     std::string out;                                                        \
     std::string expected;                                                   \
     base::FilePath src_dir;                                                 \
     PathService::Get(base::DIR_SOURCE_ROOT, &src_dir);                      \
     base::SetCurrentDirectory(src_dir);                                     \
     EXPECT_TRUE(commands::FormatFileToString(                               \
         &setup, SourceFile("//tools/gn/format_test_data/" #n ".gn"), false, \
@@ skipping 69 matching lines @@
 FORMAT_TEST(058)
 FORMAT_TEST(059)
 FORMAT_TEST(060)
 FORMAT_TEST(061)
 FORMAT_TEST(062)
 FORMAT_TEST(063)
 FORMAT_TEST(064)
 FORMAT_TEST(065)
 FORMAT_TEST(066)
 FORMAT_TEST(067)