Use shared memory for moving data over BlobChannel Mojo interface.

blimp/engine/mojo/blob_channel.mojom

Index: blimp/engine/mojo/blob_channel.mojom
diff --git a/blimp/engine/mojo/blob_channel.mojom b/blimp/engine/mojo/blob_channel.mojom
index 0960916fea10655f4e3511b8e2c1684108c08ace..7fa934148d5914a386cab5cabfff90255e384711 100644
--- a/blimp/engine/mojo/blob_channel.mojom
+++ b/blimp/engine/mojo/blob_channel.mojom
@@ -6,9 +6,12 @@ module blimp.engine.mojom;
 
 interface BlobChannel {

[dcheng, 2016/06/28 06:26:18]

If possible, a comment describing this interface would be useful (e.g. where the
service lives, what it's used for, what's in the blobs, expected
size/limitations, etc)

[Kevin M, 2016/06/28 18:18:53]

Done, though I don't want to go into too much detail here - the canonical
descriptions should be kept in blimp/net/blob_channel/*.

   // Stores the blob |id| in the BlobCache.
-  // TODO(kmarshall): Use shared memory for more efficient data transfer, see
-  // crbug.com/614564 .
-  PutBlob(string id, string data);
+  // Because the IPC channel is a shared resource and payloads can be quite
+  // large, we use shared memory to reduce channel contention and associated
+  // latency with other components/features.
+  //
+  // The buffer is released when the callee discards the handle for |data|.
+  PutBlob(string id, handle<shared_buffer> data, uint32 size);
 
   // Requests that the BlobChannel push the blob |id| over the wire.
   // The request will be ignored if the BlobChannel knows that