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

blimp/common/blob_cache/blob_cache.h

Index: blimp/common/blob_cache/blob_cache.h
diff --git a/blimp/common/blob_cache/blob_cache.h b/blimp/common/blob_cache/blob_cache.h
index 1f8a2932723640f2346321797ca98e29cbcb9f5d..e078fbd207ccae0cf64f29e3515b166b1dd76430 100644
--- a/blimp/common/blob_cache/blob_cache.h
+++ b/blimp/common/blob_cache/blob_cache.h
@@ -25,6 +25,9 @@ class BLIMP_COMMON_EXPORT BlobCache {
  public:
   virtual ~BlobCache() {}
 
+  // Gets the keys of items stored by the cache.
+  virtual std::vector<BlobId> GetCachedBlobIds() const = 0;
+
   // Returns true if there is a cache item |id|.
   virtual bool Contains(const BlobId& id) const = 0;