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Add a new cr-splitter Polymer element and use this to replace
cr.ui.Splitter in the bookmarks page. cr.ui.splitter depends on
cr.ui.define(), which will not be migrated to Polymer 3.

Also generate Polymer 3 versions of the new cr-splitter and its tests.

Fixed: 1023002
Change-Id: Ie8360353c1e67fa5fc8d2fcf3d09002621d71ba4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1910701
Commit-Queue: Rebekah Potter <rbpotter@chromium.org>
Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#714811}