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..486e2f7ddc1892ee170b1c8d50700971206b5992 100644
--- a/blimp/engine/renderer/blob_channel_sender_proxy.h
+++ b/blimp/engine/renderer/blob_channel_sender_proxy.h
@@ -21,6 +21,10 @@ namespace engine {
 class BLIMP_COMMON_EXPORT BlobChannelSenderProxy : public BlobChannelSender {
  public:
   BlobChannelSenderProxy();
+
+  // Testing constructor, used to supply a BlobChannel Mojo proxy directly.
+  explicit BlobChannelSenderProxy(mojom::BlobChannelPtr blob_channel);
+
   ~BlobChannelSenderProxy() override;
 
   // Returns true if the blob is known to exist within the Engine cache.