Reland: [Extensions] Change ProcessManager to use RenderFrameHosts

      (Reland of https://codereview.chromium.org/1037263004)

RenderViewHosts are going to go away, and things using them are blocking
OOPIs. Make ProcessManager use RenderFrameHosts.
Additionally, this takes out ~130 lines of code.

BUG=466373
(TBRing other reviewers from original patch, since no code there changed)
TBR=avi@chromium.org
TBR=asargent@chromium.org
TBR=atwilson@chromium.org
TBR=jamescook@chromium.org
TBR=dgozman@chromium.org
TBR=nick@chromium.org

Review URL: https://codereview.chromium.org/1056463005

Cr-Commit-Position: refs/heads/master@{#323803}