Add a //nogncheck anntation to GN include checker

tools/gn/variables.cc

Index: tools/gn/variables.cc
diff --git a/tools/gn/variables.cc b/tools/gn/variables.cc
index 472554ae964f57c484f687bafbc14e249c50533d..0f3ede7e181b1812e064bbab9b7a3ec549e134db 100644
--- a/tools/gn/variables.cc
+++ b/tools/gn/variables.cc
@@ -442,6 +442,27 @@ const char kCheckIncludes_Help[] =
     "  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"