Use PASS() everywhere in ppapi/tests.

ppapi/tests/test_file_io.cc

Index: ppapi/tests/test_file_io.cc
diff --git a/ppapi/tests/test_file_io.cc b/ppapi/tests/test_file_io.cc
index 5ba571a71ef12bfc4986ccba0bea89c8cc92c9dd..591d6567781d95e973694fb1eb4574c4c23f0a4c 100644
--- a/ppapi/tests/test_file_io.cc
+++ b/ppapi/tests/test_file_io.cc
@@ -113,7 +113,7 @@ std::string TestFileIO::TestOpen() {
   if (rv != PP_ERROR_FILENOTFOUND)
     return ReportError("FileIO::Open", rv);
 
-  return "";
+  PASS();
 }
 
 std::string TestFileIO::TestReadWriteSetLength() {
@@ -227,7 +227,7 @@ std::string TestFileIO::TestReadWriteSetLength() {
     return ReportMismatch("FileIO::Read", read_buffer,
                           std::string("testtest\0\0\0\0", 12));
 
-  return "";
+  PASS();
 }
 
 std::string TestFileIO::TestTouchQuery() {
@@ -281,5 +281,5 @@ std::string TestFileIO::TestTouchQuery() {
       (info.last_modified_time != last_modified_time))
     return "FileSystem::Query() has returned bad data.";
 
-  return "";
+  PASS();
 }