Introduce a RandonNumberGenerator class. Refactor the random/private_random uses in Isolate/Context.

src/isolate.h

Index: src/isolate.h
diff --git a/src/isolate.h b/src/isolate.h
index f0854e510e510c445c3a1f610ad82539d388e769..58c4f8b374d49d84536acaa8b4fcf9bc18305b07 100644
--- a/src/isolate.h
+++ b/src/isolate.h
@@ -77,6 +77,7 @@ class NoAllocationStringAllocator;
 class InnerPointerToCodeCache;
 class MarkingThread;
 class PreallocatedMemoryThread;
+class RandomNumberGenerator;
 class RegExpStack;
 class SaveContext;
 class UnicodeCache;
@@ -1125,6 +1126,8 @@ class Isolate {
 
   void* stress_deopt_count_address() { return &stress_deopt_count_; }
 
+  inline RandomNumberGenerator* random_number_generator();
+
   // Given an address occupied by a live code object, return that object.
   Object* FindCodeObject(Address a);
 
@@ -1299,6 +1302,7 @@ class Isolate {
   DateCache* date_cache_;
   unibrow::Mapping<unibrow::Ecma262Canonicalize> interp_canonicalize_mapping_;
   CodeStubInterfaceDescriptor* code_stub_interface_descriptors_;
+  RandomNumberGenerator* random_number_generator_;
 
   // True if fatal error has been signaled for this isolate.
   bool has_fatal_error_;