move protect_file_posix to posix block

Otherwise, on Windows when 'All' is built the no-op build is broken --
no files are included in the .lib so none is generated, which means on
subsequent builds ninja always reruns that step.

This is currently breaking win_chromium_compile_dbg_ng trybots.

R=pasko@chromium.org, thakis@chromium.org
BUG=424562

Review URL: https://codereview.chromium.org/981623002

Cr-Commit-Position: refs/heads/master@{#319119}

base/base.gyp

@@ -382,6 +382,8 @@
       # TODO(pasko): Remove this target when crbug.com/424562 is fixed.
       # GN: //base:protect_file_posix
       'target_name': 'protect_file_posix',
+      'conditions': [
+        ['os_posix == 1', {
           'type': 'static_library',
           'dependencies': [
             'base',
@@ -389,6 +391,10 @@
           'sources': [
             'files/protect_file_posix.cc',
           ],
+        }, {
+          'type': 'none',
+        }],
+      ],
     },
     {
       'target_name': 'base_prefs_test_support',