Clean up DelegatedFrameHost's interface to its client

Clean up DelegatedFrameHost's interface to its client

Rename client methods to start with the prefix DelegatedFrame, to indicate
to which interface they belong (not doing this had resulted in each RWHV
having IsHidden and IsVisible functions, which weren't opposites of each other).

Do not access the RenderWidgetHostImpl directly in DelegatedFrameHost, but
rather create client methods for all functions that will need to access it (and,
for most of these methods, send the IPCs directly on the RWHI, rather than
re-looking it up by its process and routing).

Clean up the interfaces used for testing.

BUG=(paying down technical debt)

Review URL: https://codereview.chromium.org/864333004

Cr-Commit-Position: refs/heads/master@{#313028}