PPAPI/NaCl: Make a gyp flag for including the untrusted IPC proxy.

This way, those of us working on the proxy switch need only add:
{
  'variables': {
    'build_ppapi_ipc_proxy_untrusted': '1',
  },
}
to "~/.gyp/include.gypi"

...and we'll be able to build our untrusted targets without editing all.gyp. For anybody who doesn't set that flag, our ppapi_proxy_untrusted.gypi file will still be parsed, but it doesn't matter if the source files build (or even exist).

BUG=116317
TEST=

Review URL: https://chromiumcodereview.appspot.com/10399068

git-svn-id: http://src.chromium.org/svn/trunk/src/build@137751 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

common.gypi

@@ -273,6 +273,11 @@
       # plugins to make call of the main thread.
       'enable_pepper_threading%': 0,
 
+      # Include the PPAPI IPC proxy for NaCl. This is a work-in-progress; this
+      # allows us to build this feature locally without it affecting others
+      # working in affected subsystems like base and ipc.
+      'build_ppapi_ipc_proxy_untrusted%': 0,
+
       # Enables use of the session service, which is enabled by default.
       # Support for disabling depends on the platform.
       'enable_session_service%': 1,
@@ -523,6 +528,7 @@
     'use_gnome_keyring%': '<(use_gnome_keyring)',
     'linux_fpic%': '<(linux_fpic)',
     'enable_pepper_threading%': '<(enable_pepper_threading)',
+    'build_ppapi_ipc_proxy_untrusted%': '<(build_ppapi_ipc_proxy_untrusted)',
     'chromeos%': '<(chromeos)',
     'use_virtual_keyboard%': '<(use_virtual_keyboard)',
     'enable_viewport%': '<(enable_viewport)',