Make transition to "postinstall" domain explicit.

When running the postinstall program, we were relying on the automatic
transition between update_engine domain and postinstall, which required
postinstall_file to be an entrypoint for such domain. This patch makes
the transition explicit by calling setexeccon(3).

Bug: 28008031
TEST=make dist; Deployed an update to edison-eng: postinstall runs as "postinstall" domain.

Change-Id: I27d404886841e0bd39589f0274eaaf98929eeac9
diff --git a/common/subprocess_unittest.cc b/common/subprocess_unittest.cc
index 5ca44e8..d5e33bb 100644
--- a/common/subprocess_unittest.cc
+++ b/common/subprocess_unittest.cc
@@ -148,6 +148,7 @@
       {kBinPath "/sh", "-c", "echo on stdout; echo on stderr >&2"},
       0,
       {},
+      nullptr,
       base::Bind(&ExpectedResults, 0, "on stdout\n")));
   loop_.Run();
 }
@@ -158,6 +159,7 @@
       {kBinPath "/sh", "-c", "echo on pipe >&3"},
       0,
       {3},
+      nullptr,
       base::Bind(&ExpectedDataOnPipe, &subprocess_, &pid, 3, "on pipe\n", 0));
   EXPECT_NE(0, pid);
 
@@ -174,10 +176,7 @@
   const vector<string> cmd = {kBinPath "/sh", "-c",
      base::StringPrintf("echo on pipe >/proc/self/fd/%d", pipe.writer)};
   EXPECT_TRUE(subprocess_.ExecFlags(
-      cmd,
-      0,
-      {},
-      base::Bind(&ExpectedResults, 1, "")));
+      cmd, 0, {}, nullptr, base::Bind(&ExpectedResults, 1, "")));
   loop_.Run();
 }