Add a mimeHandler extension API.

extensions/renderer/resources/binding.js

Index: extensions/renderer/resources/binding.js
diff --git a/extensions/renderer/resources/binding.js b/extensions/renderer/resources/binding.js
index 9804b617a4f3fb24a6f0ec26a7adbecd1a57c917..b1a3efcaca5002bb03d2918acec145572dd74e81 100644
--- a/extensions/renderer/resources/binding.js
+++ b/extensions/renderer/resources/binding.js
@@ -77,11 +77,15 @@ APIFunctions.prototype.setHandleRequestWithPromise =
         return module.createKeepAlive();
       });
       $Function.apply(customizedFunction, this, args).then(function(result) {
-        sendRequestHandler.safeCallbackApply(
-            name, {'stack': stack}, callback, [result]);
+        if (callback) {

[benwells, 2015/01/19 01:05:19]

Why is this necessary now? Do null callbacks normally get generated
automatically, or something?

[Sam McNally, 2015/01/19 04:29:57]

This is only used by APIs implemented on mojo and abortStream() is the first
with an optional callback. Optional args that are omitted turn into null (or
maybe undefined).

[benwells, 2015/01/19 04:56:54]

OK. What I am wondering is if we should make safeCallbackApply and lastError.run
check the callback instead of doing it here, but I don't think it really
matters.

+          sendRequestHandler.safeCallbackApply(name, {'stack': stack}, callback,
+                                               [result]);
+        }
       }).catch(function(error) {
-        var message = exceptionHandler.safeErrorToString(error, true);
-        lastError.run(name, message, stack, callback);
+        if (callback) {
+          var message = exceptionHandler.safeErrorToString(error, true);
+          lastError.run(name, message, stack, callback);
+        }
       }).then(function() {
         keepAlivePromise.then(function(keepAlive) {
           keepAlive.close();