Clarify proxy usage in content::FencedFrame

Code and comments in this class refer to the RenderFrameProxyHost
created to represent the inner main frame as a "proxy to the inner main
frame." This is misleading given the terminology used elsewhere (e.g.
GetProxyToParent). That is, the proxy used here is a representative *of*
the inner main frame *to* the outer frame.

There's also no need to store a pointer to the proxy as a member of
content::FencedFrame, since the only caller of
CreateProxyAndAttachToOuterFrameTree immediately accessed it.

Bug: None
Change-Id: I583f895dabd27d2b0089cc8045d3ae18f57896c9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3721413
Reviewed-by: Alex Moshchuk <alexmos@chromium.org>
Commit-Queue: Kevin McNee <mcnee@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1017881}