[Extensions OOPI] Convert extension functions to use RenderFrameHosts

[Extensions OOPI] Convert extension functions to use RenderFrameHosts

Make ExtensionHostMsg_Request/Response send to RenderFrameHosts/RenderFrames
so that OOPI work with extension api calls.

Also update ExtensionFunctions to only have RenderFrameHosts set, and remove
the |render_view_host_| parameter, and update ExtensionFunction's
internals to use RenderFrameHost.
Note that this doesn't update all the uses of
ExtensionFunction::render_view_host() (of which there are 100+).

BUG=498017

Review URL: https://codereview.chromium.org/1182493002

Cr-Commit-Position: refs/heads/master@{#334185}