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

blimp/engine/renderer/blob_channel_sender_proxy.h

Index: blimp/engine/renderer/blob_channel_sender_proxy.h
diff --git a/blimp/engine/renderer/blob_channel_sender_proxy.h b/blimp/engine/renderer/blob_channel_sender_proxy.h
index d34e482459821a550b0341e15ef83613107680ea..1ee09f7ac10f79d8d8df34ba2ddbc2cdd59b3b18 100644
--- a/blimp/engine/renderer/blob_channel_sender_proxy.h
+++ b/blimp/engine/renderer/blob_channel_sender_proxy.h
@@ -5,10 +5,14 @@
 #ifndef BLIMP_ENGINE_RENDERER_BLOB_CHANNEL_SENDER_PROXY_H_
 #define BLIMP_ENGINE_RENDERER_BLOB_CHANNEL_SENDER_PROXY_H_
 
+#include <map>
 #include <memory>
 #include <string>
+#include <unordered_map>
+#include <vector>
 
 #include "base/containers/hash_tables.h"
+#include "base/memory/weak_ptr.h"
 #include "blimp/common/blimp_common_export.h"
 #include "blimp/engine/mojo/blob_channel.mojom.h"
 #include "blimp/net/blob_channel/blob_channel_sender.h"
@@ -20,6 +24,14 @@ namespace engine {
 // process.
 class BLIMP_COMMON_EXPORT BlobChannelSenderProxy : public BlobChannelSender {
  public:
+  // Asynchronously request the set of cache keys from the browser process.
+  // Knowledge of the browser's cache allows the renderer to avoid re-encoding
+  // and re-transferring image assets that are already tracked by the
+  // browser-side cache.
+  //
+  // If |this| is used in the brief time before the response arrives,
+  // the object will continue to work, but false negatives will be returned
+  // for IsInEngineCache() and IsInClientCache().
   BlobChannelSenderProxy();
 
   ~BlobChannelSenderProxy() override;
@@ -34,10 +46,14 @@ class BLIMP_COMMON_EXPORT BlobChannelSenderProxy : public BlobChannelSender {
   bool IsInClientCache(const std::string& id) const;
 
   // BlobChannelSender implementation.
+  std::vector<CacheStateEntry> GetCachedBlobIds() const override;
   void PutBlob(const BlobId& id, BlobDataPtr data) override;
   void DeliverBlob(const BlobId& id) override;
 
  private:
+  void OnGetCacheStateComplete(
+      const std::unordered_map<std::string, bool>& cache_state);
+
   // Testing constructor, used to supply a BlobChannel Mojo proxy directly.
   explicit BlobChannelSenderProxy(mojom::BlobChannelPtr blob_channel);
 
@@ -50,6 +66,8 @@ class BLIMP_COMMON_EXPORT BlobChannelSenderProxy : public BlobChannelSender {
   // image encoding and transferral if the content is already in the system.
   base::hash_map<std::string, bool> replication_state_;
 
+  base::WeakPtrFactory<BlobChannelSenderProxy> weak_factory_;
+
   DISALLOW_COPY_AND_ASSIGN(BlobChannelSenderProxy);
 };