Move overrideEncoding to FrameHost

Currently, it lives on DocumentLoader, which requires it to be plumbed down through FrameLoader::reload() and propagated all over to ensure it is properly set on new DocumentLoaders. There may have been a time in the past when different frames could have different override encodings, but that doesn't appear to be the case anymore.

Also, remove the default value for the ReloadPolicy argument on FrameLoader::reload(), as only one callsite makes use of it.

BUG=

Review URL: https://codereview.chromium.org/601863003

git-svn-id: svn://svn.chromium.org/blink/trunk@182872 bbb929c8-8fbe-4397-9dbb-9b2b20218538