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

runtime/bin/crypto_fuchsia.cc

Index: runtime/bin/crypto_fuchsia.cc
diff --git a/runtime/bin/crypto_fuchsia.cc b/runtime/bin/crypto_fuchsia.cc
index 9423f7cdf9423a3bed545b2f051bb42dae635dc1..04c5f2254760ee7b279625a1190d32a34b5ddcb4 100644
--- a/runtime/bin/crypto_fuchsia.cc
+++ b/runtime/bin/crypto_fuchsia.cc
@@ -7,20 +7,23 @@
 
 #include "bin/crypto.h"
 
+#include <magenta/syscalls.h>
+
 namespace dart {
 namespace bin {
 
 bool Crypto::GetRandomBytes(intptr_t count, uint8_t* buffer) {
-  uint32_t num;
   intptr_t read = 0;
   while (read < count) {
-    if (rand_r(&num) != 0) {
+    const intptr_t remaining = count - read;
+    const intptr_t len =
+        (MX_CPRNG_DRAW_MAX_LEN < remaining) ? MX_CPRNG_DRAW_MAX_LEN
+                                            : remaining;
+    const mx_ssize_t res = mx_cprng_draw(buffer + read, len);
+    if (res == ERR_INVALID_ARGS) {
       return false;
     }
-    for (int i = 0; i < 4 && read < count; i++) {
-      buffer[read] = num >> (i * 8);
-      read++;
-    }
+    read += res;
   }
   return true;
 }