Use PASS() everywhere in ppapi/tests.

ppapi/tests/test_buffer.cc

Index: ppapi/tests/test_buffer.cc
diff --git a/ppapi/tests/test_buffer.cc b/ppapi/tests/test_buffer.cc
index bcef727da274eb86325911b91c6356d6f9aa1af8..75c3bbbc10c45b640fdd5be57c360cd6065b564e 100644
--- a/ppapi/tests/test_buffer.cc
+++ b/ppapi/tests/test_buffer.cc
@@ -30,7 +30,7 @@ std::string TestBuffer::TestInvalidSize() {
   if (!zero_size.is_null())
     return "Zero size accepted";
 
-  return "";
+  PASS();
 }
 
 std::string TestBuffer::TestInitToZero() {
@@ -48,7 +48,7 @@ std::string TestBuffer::TestInitToZero() {
       return "Buffer isn't entirely zero";
   }
 
-  return "";
+  PASS();
 }
 
 std::string TestBuffer::TestIsBuffer() {
@@ -72,6 +72,6 @@ std::string TestBuffer::TestIsBuffer() {
   if (!buffer_interface_->IsBuffer(buffer.pp_resource()))
     return "Buffer should be identified as a buffer";
 
-  return "";
+  PASS();
 }