Pepper: Make Mojo test more strict.

Mojo's MessagePipe constructor uses assert() to check that
MojoCreateMessagePipe returns OK. This change makes the test explicitly
check the return value of MojoCreateMessagePipe, giving more correct
test behavior for Release builds.

Review URL: https://codereview.chromium.org/897093004

Cr-Commit-Position: refs/heads/master@{#315168}

ppapi/tests/mojo/test_mojo.cc

@@ -18,6 +18,8 @@ void TestMojo::RunTests(const std::string& filter) {
 }
 
 std::string TestMojo::TestCreateMessagePipe() {
-  mojo::MessagePipe pipe;
+  MojoHandle h0;
+  MojoHandle h1;
+  ASSERT_EQ(MOJO_RESULT_OK, MojoCreateMessagePipe(NULL, &h0, &h1));
   PASS();
 }