Make cc::SurfaceId unguessable

cc/surfaces/surface_id_allocator.cc

Index: cc/surfaces/surface_id_allocator.cc
diff --git a/cc/surfaces/surface_id_allocator.cc b/cc/surfaces/surface_id_allocator.cc
index d71467562e1ca21fbae3ac651fe2aaf1f847177d..d64306cd3feb16266f821a5a667f0016c8239db0 100644
--- a/cc/surfaces/surface_id_allocator.cc
+++ b/cc/surfaces/surface_id_allocator.cc
@@ -6,6 +6,7 @@
 
 #include <stdint.h>
 
+#include "base/rand_util.h"
 #include "cc/surfaces/surface_manager.h"
 
 namespace cc {
@@ -26,14 +27,10 @@ SurfaceIdAllocator::~SurfaceIdAllocator() {
 }
 
 SurfaceId SurfaceIdAllocator::GenerateId() {
-  SurfaceId id(static_cast<uint64_t>(id_namespace_) << 32 | next_id_);
+  uint64_t nonce = base::RandUint64();

[Tom Sepez, 2016/05/24 22:35:45]

Can we use the random number generator in crypto?  This one isn't advertised as
being secure (though it is in practice).

[Fady Samuel, 2016/05/25 00:04:29]

I'd prefer not to have cc/surfaces depend on all of crypto just for random.cc.
Perhaps we can split that off to a separate target in a future CL.

+  SurfaceId id(id_namespace_, nonce, next_id_);
   next_id_++;
   return id;
 }
 
-// static
-uint32_t SurfaceIdAllocator::NamespaceForId(SurfaceId id) {
-  return id.id >> 32;
-}
-
 }  // namespace cc