Fixing appview documentation.

Review URL: https://codereview.chromium.org/1032913002

Cr-Commit-Position: refs/heads/master@{#322239}

chrome/common/extensions/api/appview_tag.idl

@@ -14,7 +14,7 @@ namespace appviewTag {
   // request.
   dictionary EmbedRequest {
     // The ID of the app that sent the embedding request.
-    string embedderId;
+    DOMString embedderId;
 
     // Optional developer specified data that the app to be embedded can use
     // when making an embedding decision.