Snapshot clean ups

- Don't call eval() in mock_runtime - not allowed - see js2c.py.
- Don't use v8_use_external_startup_data
- Move MakeSnapshot to snapshot_creator.cc
- Use logging.h in from_snapshot.cc

deno2/js/mock_runtime.js

@@ -1,6 +1,5 @@
 // A simple runtime that doesn't involve typescript or protobufs to test
 // libdeno. Invoked by mock_runtime_test.cc
-const window = eval("this");
 
 function assert(cond) {
   if (!cond) throw Error("mock_runtime.js assert failed");
@@ -68,7 +67,6 @@ function DoubleSubFails() {
   deno.sub((channel, msg) => assert(false));
 }
 
-
 // The following join has caused SnapshotBug to segfault when using kKeep.
 [].join("");