Fix external-snapshot startup when snapshot is missing, but natives source is available

src/isolate.h

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef V8_ISOLATE_H_
 #define V8_ISOLATE_H_
 
 #include <queue>
 #include "include/v8-debug.h"
 #include "src/allocation.h"
@@ skipping 968 matching lines @@
   void SetData(uint32_t slot, void* data) {
     DCHECK(slot < Internals::kNumIsolateDataSlots);
     embedder_data_[slot] = data;
   }
   void* GetData(uint32_t slot) {
     DCHECK(slot < Internals::kNumIsolateDataSlots);
     return embedder_data_[slot];
   }
 
   bool serializer_enabled() const { return serializer_enabled_; }
-  bool snapshot_available() const { return snapshot_blob_ != NULL; }
+  bool snapshot_available() const {
+    return snapshot_blob_ != NULL && snapshot_blob_->raw_size != 0;

[vogelheim, 2015/04/01 17:02:13]

When does this happen?

I'm a bit surprised. I don't think an embedder should give us an empty snapshot;
they should just not give us a snapshot at all. In the static initialization
case we at least DCHECK for this situation.

ref:
https://code.google.com/p/chromium/codesearch#chromium/src/v8/src/snapshot/sn...

[Erik Corry, 2015/04/01 17:07:29]

This isn't the embedder giving us an empty snapshot, it's our own default
snapshot, which has a null data field.

[vogelheim, 2015/04/01 17:21:09]

Oh. :-(

I guess it shouldn't do that; but since it does we'll need the check.

+  }
 
   bool IsDead() { return has_fatal_error_; }
   void SignalFatalError() { has_fatal_error_ = true; }
 
   bool use_crankshaft() const;
 
   bool initialized_from_snapshot() { return initialized_from_snapshot_; }
 
   double time_millis_since_init() {
     return base::OS::TimeCurrentMillis() - time_millis_at_init_;
@@ skipping 567 matching lines @@
   }
 
   EmbeddedVector<char, 128> filename_;
   FILE* file_;
   int scope_depth_;
 };
 
 } }  // namespace v8::internal
 
 #endif  // V8_ISOLATE_H_