Updating documentation.

This CL adds the embedderId parameter to appview documentation.

Review URL: https://codereview.chromium.org/1010253003

Cr-Commit-Position: refs/heads/master@{#321458}

chrome/common/extensions/api/appview_tag.idl

@@ -13,6 +13,9 @@ namespace appviewTag {
   // allow the embedding and what to embed based on the embedder making the
   // request.
   dictionary EmbedRequest {
+    // The ID of the app that sent the embedding request.
+    string embedderId;
+
     // Optional developer specified data that the app to be embedded can use
     // when making an embedding decision.
     object data;