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This CL updates the description of SiteInstance and the various
process models now that Site Isolation has launched. It also
separates the notions of the ideal abstraction (agent cluster
represented by principal and browsing context group) from the
practical implementation (where the definition of "site" varies
by process model).

BUG=1015882

Change-Id: I93a8d495ea3ab8dc69a5446d83dd841ee9fae64c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2256605
Commit-Queue: Charlie Reis <creis@chromium.org>
Reviewed-by: Kouhei Ueno <kouhei@chromium.org>
Reviewed-by: Alex Moshchuk <alexmos@chromium.org>
Cr-Commit-Position: refs/heads/master@{#783041}