We already know how to talk to the network_service from Dart
via fetch.dart.  Might as well use that for Image loading
as well insetad of having ImageLoader do it.

As part of this I've renamed *ImageLoader to *ImageDecoder
and moved all the image loading logic into Dart.  This required
me to teach the idl system about mojo handles so that I could
pass the resulting MojoHandle from fetch.dart up through to
ImageDecoder.

R=abarth@chromium.org, jackson@google.com, hansmuller@google.com

Review URL: https://codereview.chromium.org/1173703002.