Add Mojo IPC for seeding new Renderer with Browser's cached blob state.

blimp/engine/mojo/blob_channel.mojom

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 module blimp.engine.mojom;
 
+struct CacheStateEntry {
+  // The non-human-readable ID of the cache item.
+  string id;
+
+  // True if the cached item exists on the remote end of the BlobChannel.
+  bool was_delivered;
+};
+
 interface BlobChannel {
+  // Gets the list of cache keys and their replication status in the
+  // browser process' BlobCache.
+  GetCachedBlobIds() => (array<CacheStateEntry> cache_state);
+
   // Stores the blob |id| in the BlobCache.
   // TODO(kmarshall): Use shared memory for more efficient data transfer, see
   // crbug.com/614564 .
   PutBlob(string id, string data);
 
   // Requests that the BlobChannel push the blob |id| over the wire.
   // The request will be ignored if the BlobChannel knows that
   // the client already has the data.
   DeliverBlob(string id);
 };