Add a mimeHandler extension API.

extensions/common/api/mime_handler.mojom

Index: extensions/common/api/mime_handler.mojom
diff --git a/extensions/common/api/mime_handler.mojom b/extensions/common/api/mime_handler.mojom
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+// Copyright 2015 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+module extensions.mime_handler;
+
+// Information about a stream.
+struct StreamInfo {
+  // The MIME type of the intercepted URL request.
+  string mime_type;
+
+  // The original URL that was intercepted.
+  string original_url;
+
+  // The URL that the stream can be read from.
+  string stream_url;
+
+  // The ID of the tab that opened the stream. If the stream is not opened in a
+  // tab, it will be -1.
+  int32 tab_id;
+
+  // The HTTP response headers of the intercepted request stored as a dictionary
+  // mapping header name to header value. If a header name appears multiple
+  // times, the header values are merged in the dictionary and separated by a
+  // ",".
+  map<string, string> response_headers;
+
+  // Whether the stream is embedded within another document.
+  bool embedded;
+};
+
+interface MimeHandlerService {
+  // Returns information about the stream associated with this service instance.
+  // If the stream has been aborted, |stream_info| will be null.
+  GetStreamInfo() => (StreamInfo? stream_info);
+
+  // Aborts the stream associated with this service instance. This is an
+  // idempotent operation.
+  AbortStream() => ();
+};