nacl: Be more consistent about setting nacl_untrusted_build on targets that use untrusted.gypi

The motivation is that I want to add a certain include_dir to all targets (to
use libc++ on OS X), _except_ for NaCl targets, since these have their own
bundled C++ standard library already.  Checking for nacl_untrusted_build almost
works, but a handful nacl targets didn't set that yet.  Fix this.  No intended
behavior change.

BUG=400091

Review URL: https://codereview.chromium.org/1332163005

Cr-Commit-Position: refs/heads/master@{#348815}

ppapi/ppapi_nacl.gyp

@@ -18,6 +18,7 @@
       'variables': {
         'nlib_target': 'libppapi_cpp.a',
         'nso_target': 'libppapi_cpp.so',
+        'nacl_untrusted_build': 1,
         'build_glibc': 1,
         'build_newlib': 1,
         'build_pnacl_newlib': 1,
@@ -33,6 +34,7 @@
       'type': 'none',
       'variables': {
         'nlib_target': 'libppapi_gles2.a',
+        'nacl_untrusted_build': 1,
         'nso_target': 'libppapi_gles2.so',
         'build_glibc': 1,
         'build_newlib': 1,
@@ -76,6 +78,7 @@
           'GL_GLEXT_PROTOTYPES',
         ],
         'nexe_target': 'ppapi_nacl_tests',
+        'nacl_untrusted_build': 1,
         'build_newlib': 1,
         'include_dirs': [
           'lib/gl/include',

ppapi/ppapi_nacl_test_common.gypi

@@ -14,6 +14,7 @@
     # variables overriden inside a target_condition block in this file will not
     # affect the values in untrusted.gypi.
     'variables': {
+      'nacl_untrusted_build': 1,
       'test_files': [],
       'generate_nmf%': 1,
       'nacl_newlib_out_dir': '<(PRODUCT_DIR)/>(nexe_destination_dir)/newlib',

third_party/libjingle/libjingle_nacl.gyp

@@ -301,6 +301,7 @@
       'type': 'none',
       'variables': {
         'nlib_target': 'libjingle_p2p_constants_nacl.a',
+        'nacl_untrusted_build': 1,
         'build_glibc': 0,
         'build_newlib': 1,
         'build_pnacl_newlib': 1,

third_party/modp_b64/modp_b64_nacl.gyp

@@ -15,6 +15,7 @@
         'build_glibc': 0,
         'build_newlib': 1,
         'build_pnacl_newlib': 1,
+        'nacl_untrusted_build': 1,
       },
       'sources': [
         'modp_b64.cc',