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This change removes some WidgetDelegate overrides in ImeWindowView
and replaces them with setter calls. It also slightly restructures
ImeWindow to make it clear that both title and icon are const for
the lifetime of the ImeWindow.

Bug: 1075649
Change-Id: I1d0a9117bc45b1c56362627dbc4f98b20beb7954
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2207970
Commit-Queue: Elly Fong-Jones <ellyjones@chromium.org>
Reviewed-by: Peter Kasting <pkasting@chromium.org>
Cr-Commit-Position: refs/heads/master@{#769851}