Add missing dependencies on mojo_system_placeholder.

These targets use headers from mojo_system and thus need to obtain the settings
that the mojo_system target passes on to its direct dependents. As they cannot
depend on mojo_system or mojo_system_impl, they instead are made to depend on
mojo_system_placeholder, which is currently empty but will in
the near future have settings to pass on to its direct dependents.

R=jamesr@chromium.org, ncbray@chromium.org, teravest@chromium.org

Review URL: https://codereview.chromium.org/840543004

Cr-Commit-Position: refs/heads/master@{#310463}

mojo/mojo_nacl.gyp

@@ -49,6 +49,9 @@
           'sources': [
             '<(monacl_codegen_dir)/mojo_syscall.cc',
           ],
+          'dependencies': [
+            'mojo_public.gyp:mojo_system_placeholder',
+          ],
         },
         {
           'target_name': 'monacl_sel',
@@ -96,6 +99,9 @@
               'sources': [
                 '<(monacl_codegen_dir)/mojo_syscall.cc',
               ],
+              'dependencies': [
+                'mojo_public.gyp:mojo_system_placeholder',
+              ],
               'configurations': {
                 'Common_Base': {
                   'msvs_target_platform': 'x64',

mojo/mojo_nacl_untrusted.gyp

@@ -31,6 +31,7 @@
           ],
           'dependencies': [
             'mojo_nacl.gyp:monacl_codegen',
+            'mojo_public.gyp:mojo_system_placeholder',
           ],
         },
         {

ppapi/ppapi_tests_mojo.gyp

@@ -41,6 +41,7 @@
       'dependencies': [
         '../mojo/mojo_nacl_untrusted.gyp:libmojo',
         '../mojo/mojo_nacl.gyp:monacl_codegen',
+        '../mojo/mojo_public.gyp:mojo_system_placeholder',
         '../native_client/src/untrusted/nacl/nacl.gyp:imc_syscalls_lib',
         'native_client/native_client.gyp:ppapi_lib',
         'ppapi_nacl.gyp:ppapi_cpp_lib',