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-Deletes the following unused classes: BookmarkElevatedToolbar, LegacyBookmarkParentFolderItem,
LegacyBookmarkTextFieldItem.
-Creates a BookmarkUIConstants file.
-Adds a Legacy prefix to BookmarkCells, to prepare for refresh cells.

Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Change-Id: I35662f194eaada4f95cbf46b636426cc98767ca4
Reviewed-on: https://chromium-review.googlesource.com/1049064Reviewed-by: Rohit Rao <rohitrao@chromium.org>
Commit-Queue: Sergio Collazos <sczs@chromium.org>
Cr-Commit-Position: refs/heads/master@{#557541}