Add optional public header checking to GN build

tools/gn/command_gen.cc

Index: tools/gn/command_gen.cc
diff --git a/tools/gn/command_gen.cc b/tools/gn/command_gen.cc
index 139e0dca0f8d0f7e2e965f7ae661c91568ae8fa6..b8e5e0fbb81d731b8c48bc106c34704c2fe04b0f 100644
--- a/tools/gn/command_gen.cc
+++ b/tools/gn/command_gen.cc
@@ -22,6 +22,8 @@ namespace {
 // Suppress output on success.
 const char kSwitchQuiet[] = "q";
 
+const char kSwitchCheck[] = "check";
+
 void BackgroundDoWrite(const Target* target,
                        const Toolchain* toolchain,
                        const std::vector<const Item*>& deps_for_visibility) {
@@ -103,6 +105,9 @@ int RunGen(const std::vector<std::string>& args) {
   if (!setup->DoSetup(args[0]))
     return 1;
 
+  if (CommandLine::ForCurrentProcess()->HasSwitch(kSwitchCheck))
+    setup->set_check_public_headers(true);
+
   // Cause the load to also generate the ninja files for each target. We wrap
   // the writing to maintain a counter.
   base::subtle::Atomic32 write_counter = 0;