Stop having two URLLoaderFactories in RendererBlinkPlatformImpl

RendererBlinkPlatformImpl has one InterfacePtr<URLLoaderFactory> and one
AssociatedInterfacePtr<URLLoaderFactory> but only one instance is needed
at one time: They are used in the same way, but held differently because
of the type difference. This CL introduces a container type to hold them.

BUG=712913
R=kinuko@chromium.org, jam@chromium.org

Review-Url: https://codereview.chromium.org/2858503002
Cr-Commit-Position: refs/heads/master@{#470926}
4 files changed