Make NaCl IPC-based PPAPI proxy build on ARM.

base/base_untrusted.gyp

Index: base/base_untrusted.gyp
===================================================================
--- base/base_untrusted.gyp	(revision 152814)
+++ base/base_untrusted.gyp	(working copy)
@@ -7,7 +7,7 @@
     'chromium_code': 1,
   },
   'includes': [
-    '../native_client/build/untrusted.gypi',
+    '../build/common_untrusted.gypi',
     'base.gypi',
   ],
   'conditions': [
@@ -23,13 +23,20 @@
             'build_glibc': 1,
             'build_newlib': 1,
             'sources': [
-              'atomicops_internals_x86_gcc.cc',
               'string16.cc',
               'sync_socket_nacl.cc',
               'third_party/nspr/prtime.cc',
               'time_posix.cc',
             ],
           },
+          'conditions': [
+            # This is needed for third_party/nspr/prtime.cc
+            ['target_arch=="arm"', {
+              'defines': [
+                '__arm__',
+              ],
+            }],
+          ],
           'dependencies': [
             '<(DEPTH)/native_client/tools.gyp:prep_toolchain',
           ],