Refactor around start up of the plugin.

This CL has some refactoring to prepare implementation of EmbedderService.
1) Moves irt_ppapi.c, plugin_main_irt.cc and plugin_main_irt.h to
   ppapi/nacl_irt.
2) Introduces irt_start.cc to handle startup of the plugin in SFI-mode.
   The file is extracted from irt_ppapi.cc. The main purpose of this separation
   is to avoid conflicting unistd.h. irt_ppapi.cc needs to include
   native_client/src/trusted/service_runtime/include/sys/unistd.h.
   However, start up code needs to include base/at_exit.h which indirectly
   include unistd.h. As a result they conflict, and
   NACL_ABI__SC_NACL_PNACL_MODE sometimes can be used, but sometimes not.
3) Moves AtExitManager, IOThread creation and global FD setting to
   irt_start.cc and plugin_startup.cc.

TEST=Ran trybots
BUG=358431

Review URL: https://codereview.chromium.org/225093006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@262649 0039d316-1c4b-4281-b951-d872f2087c98
diff --git a/ppapi/ppapi_proxy.gypi b/ppapi/ppapi_proxy.gypi
index 49e2232..11eb697 100644
--- a/ppapi/ppapi_proxy.gypi
+++ b/ppapi/ppapi_proxy.gypi
@@ -244,9 +244,15 @@
         'target_conditions': [
           ['>(nacl_untrusted_build)==1', {
             'sources': [
-              'proxy/irt_ppapi.c',
-              'proxy/plugin_main_irt.cc',
-              'proxy/plugin_main_irt.h',
+              'nacl_irt/irt_ppapi.cc',
+              'nacl_irt/irt_ppapi.h',
+              'nacl_irt/irt_start.cc',
+              'nacl_irt/plugin_main.cc',
+              'nacl_irt/plugin_main.h',
+              'nacl_irt/plugin_startup.cc',
+              'nacl_irt/plugin_startup.h',
+              'nacl_irt/ppapi_dispatcher.cc',
+              'nacl_irt/ppapi_dispatcher.h',
             ],
             'sources!': [
               'proxy/audio_input_resource.cc',