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Before out-of-process iframes, frame owners (i.e. HTMLFrameOwnerElement)
were retained by the DOM tree of the parent frame. However, remote
frames don't have a DOM tree to keep the frame owners alive. Instead,
remote frames used a map to keep the frame owners live while the
corresponding child frame was still in the frame tree.

However, this map was never updated when frames were swapped, so frame
owners would trivially leak after a frame swap. To simplify the code,
just eliminate the map and have remote frame owners keep themselves
live. With Oilpan, there's actually no need to do anything special:
Frame already has a strong reference to the FrameOwner.

Without Oilpan, it is a bit more complicated: a remote frame owner
manually refs and unrefs itself when attached to/detached from a content
frame.

BUG=none

Review URL: https://codereview.chromium.org/1807033003

Cr-Commit-Position: refs/heads/master@{#381717}