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This renames SiteSettingsClient to SiteSettingsDelegate to match other
similar interfaces, and merges SiteSettingsHelpClient and
WebappSettingsClient into SiteSettingsDelegate so there's only one
interface for embedders to implement.

Things were originally split into multiple interfaces to help group
methods together and make it easier to reason about. Whether or not that
actually worked is debatable, but now that we've removed the need for
multiple of these sub-interfaces, the 2 that remain should be merged
into the main one.

Bug: 1077007
Change-Id: Ia02d8e47d38d1b4878b4a4d0cda5ccbb74de6352
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2594441
Commit-Queue: Robbie McElrath <rmcelrath@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Natalie Chouinard <chouinard@chromium.org>
Reviewed-by: Finnur Thorarinsson <finnur@chromium.org>
Cr-Commit-Position: refs/heads/master@{#840305}