Add a //nogncheck anntation to GN include checker

tools/gn/variables.cc

 // Copyright (c) 2013 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 "tools/gn/variables.h"
 
 namespace variables {
 
 // Built-in variables ----------------------------------------------------------
 
@@ skipping 424 matching lines @@
     "check_includes: [boolean] Controls whether a target's files are checked.\n"
     "\n"
     "  When true (the default), the \"gn check\" command (as well as\n"
     "  \"gn gen\" with the --check flag) will check this target's sources\n"
     "  and headers for proper dependencies.\n"
     "\n"
     "  When false, the files in this target will be skipped by default.\n"
     "  This does not affect other targets that depend on the current target,\n"
     "  it just skips checking the includes of the current target's files.\n"
     "\n"
+    "Controlling includes individually\n"
+    "\n"
+    "  If only certain includes are problematic, you can annotate them\n"
+    "  individually rather than disabling header checking on an entire\n"
+    "  target. Add the string \"nogncheck\" to the include line:\n"
+    "\n"
+    "    #include \"foo/something_weird.h\"  // nogncheck (bug 12345)\n"
+    "\n"
+    "  It is good form to include a reference to a bug (if the include is\n"
+    "  improper, or some other comment expressing why the header checker\n"
+    "  doesn't work for this particular case.\n"
+    "\n"
+    "  The most common reason to need \"nogncheck\" is conditional includes.\n"
+    "  The header checker does not understand the preprocessor, so may flag\n"
+    "  some includes as improper even if the dependencies and #defines are\n"
+    "  always matched correctly:\n"
+    "\n"
+    "    #if defined(ENABLE_DOOM_MELON)\n"
+    "    #include \"doom_melon/beam_controller.h\"  // nogncheck\n"
+    "    #endif\n"
+    "\n"
     "Example\n"
     "\n"
     "  source_set(\"busted_includes\") {\n"
     "    # This target's includes are messed up, exclude it from checking.\n"
     "    check_includes = false\n"
     "    ...\n"
     "  }\n";
 
 const char kCompleteStaticLib[] = "complete_static_lib";
 const char kCompleteStaticLib_HelpShort[] =
@@ skipping 745 matching lines @@
     INSERT_VARIABLE(Sources)
     INSERT_VARIABLE(Testonly)
     INSERT_VARIABLE(Visibility)
   }
   return info_map;
 }
 
 #undef INSERT_VARIABLE
 
 }  // namespace variables