Preparation steps for adding speculative renderer creation.

Generalized the RenderFrameHost and WebUI creation methods to allow for the
following step of speculatively creating renderers in browser side navigations
for the PlzNavigate project (spliting off of crrev.com/701953006).

BUG=376094

TBR=creis@chromium.org

Review URL: https://codereview.chromium.org/753173002

Cr-Commit-Position: refs/heads/master@{#305989}
5 files changed