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

blimp/engine/mojo/blob_channel.mojom

Index: blimp/engine/mojo/blob_channel.mojom
diff --git a/blimp/engine/mojo/blob_channel.mojom b/blimp/engine/mojo/blob_channel.mojom
index 0960916fea10655f4e3511b8e2c1684108c08ace..a61fb92848304c844e5889d676ddc12e076910ea 100644
--- a/blimp/engine/mojo/blob_channel.mojom
+++ b/blimp/engine/mojo/blob_channel.mojom
@@ -4,7 +4,19 @@
 
 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 .