ServiceWorkerSubresourceLoader and ServiceWorkerURLLoader have a lot of common code. We should factor out the common code so we don't have so much copy/paste coding.

Note that ServiceWorkerSubresourceLoader lives in content/child (code that's executed in a renderer process), while ServiceWorkerURLLoader lives in content/browser (code that's executed in the browser process). The common code will have to live in the common place, which is called content/common.

There are two rough approaches to take:
1) Create a common base class that the two classes inherit from.
2) Create a common helper class (or classes) that the two classes can use.

In this case, I would probably attempt both approaches and see which looks to fit better, but my instinct is usually to go with 2).