Enable Oilpan for ImageBitmapFactories::ImageBitmapLoader by default.

We can't enable Oilpan for ImageBitmapFactories, another class in
modules/imagebitmap/, because it's a Supplement of DOMWindow or
WorkerGlobalScope.

BUG=340522

Review URL: https://codereview.chromium.org/323293004

git-svn-id: svn://svn.chromium.org/blink/trunk@176252 bbb929c8-8fbe-4397-9dbb-9b2b20218538