Fuchsia: Use low-level prng call, add test, update test runner.

runtime/platform/text_buffer.cc

Index: runtime/platform/text_buffer.cc
diff --git a/runtime/platform/text_buffer.cc b/runtime/platform/text_buffer.cc
index 9cbb5773da14daaf8dd5e8ddcfb2703070bce410..5b4cc05f8021417e0d3b290759a177aeb5aa8717 100644
--- a/runtime/platform/text_buffer.cc
+++ b/runtime/platform/text_buffer.cc
@@ -57,6 +57,7 @@ void TextBuffer::AddRaw(const uint8_t* buffer,
   buf_[msg_len_] = '\0';
 }
 
+
 intptr_t TextBuffer::Printf(const char* format, ...) {
   va_list args;
   va_start(args, format);
@@ -79,6 +80,7 @@ intptr_t TextBuffer::Printf(const char* format, ...) {
   return len;
 }
 
+
 // Write a UTF-32 code unit so it can be read by a JSON parser in a string
 // literal. Use official encoding from JSON specification. http://json.org/
 void TextBuffer::EscapeAndAddCodeUnit(uint32_t codeunit) {
@@ -119,6 +121,7 @@ void TextBuffer::EscapeAndAddCodeUnit(uint32_t codeunit) {
   }
 }
 
+
 // Write an incomplete UTF-16 code unit so it can be read by a JSON parser in a
 // string literal.
 void TextBuffer::EscapeAndAddUTF16CodeUnit(uint16_t codeunit) {