Use shared memory for moving data over BlobChannel Mojo interface.

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 e219ef06c35e7ff5fda51bed6f00343a4bd8ceb7..d34e482459821a550b0341e15ef83613107680ea 100644
--- a/blimp/engine/renderer/blob_channel_sender_proxy.h
+++ b/blimp/engine/renderer/blob_channel_sender_proxy.h
@@ -21,8 +21,12 @@ namespace engine {
 class BLIMP_COMMON_EXPORT BlobChannelSenderProxy : public BlobChannelSender {
  public:
   BlobChannelSenderProxy();
+
   ~BlobChannelSenderProxy() override;
 
+  static std::unique_ptr<BlobChannelSenderProxy> CreateForTest(
+      mojom::BlobChannelPtr blob_channel);
+
   // Returns true if the blob is known to exist within the Engine cache.
   bool IsInEngineCache(const std::string& id) const;
 
@@ -34,6 +38,9 @@ class BLIMP_COMMON_EXPORT BlobChannelSenderProxy : public BlobChannelSender {
   void DeliverBlob(const BlobId& id) override;
 
  private:
+  // Testing constructor, used to supply a BlobChannel Mojo proxy directly.
+  explicit BlobChannelSenderProxy(mojom::BlobChannelPtr blob_channel);
+
   // BlobChannel Mojo IPC stub, used for delivering blobs to the browser
   // process.
   mojom::BlobChannelPtr blob_channel_;