Use shared memory for moving data over BlobChannel Mojo interface.

blimp/net/blob_channel/blob_channel_sender.h

Index: blimp/net/blob_channel/blob_channel_sender.h
diff --git a/blimp/net/blob_channel/blob_channel_sender.h b/blimp/net/blob_channel/blob_channel_sender.h
index ec121c8f896c2e4bd82494850ff19da1928c5227..58c2572334d42344daca345c9ef7c866f92bdba7 100644
--- a/blimp/net/blob_channel/blob_channel_sender.h
+++ b/blimp/net/blob_channel/blob_channel_sender.h
@@ -10,6 +10,9 @@
 
 namespace blimp {
 
+// Blob size upper limit, for abuse prevention.
+const size_t kMaxBlobSizeBytes = 10 * 1024 * 1024;
+
 class BLIMP_NET_EXPORT BlobChannelSender {
  public:
   virtual ~BlobChannelSender() {}