Add the ability to override WebContentsBindingSet binders for testing

WebContentsBindingSet and WebContentsFrameBindingSet<T> provide a simple
way of binding Channel-associated interfaces which require RenderFrameHost
context for dispatch.

Currently there's no simple way to override incoming request handlers
for integration testing. This adds that.

BUG=577685

Review-Url: https://codereview.chromium.org/2752283004
Cr-Commit-Position: refs/heads/master@{#458231}
6 files changed