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There are a number of static downcasts in ShelfView that are *probably*
correct, but there is no clear proof that they are casting to the
correct type. This patch adds explicit CHECKs before every static_cast
to ensure that we are not casting from an incompatible dynamic type.

drag_view_ now has type ShelfButton*, so it is downcast on assignment,
rather than on many of its uses.

BUG=418460

Review URL: https://codereview.chromium.org/614653002

Cr-Commit-Position: refs/heads/master@{#297335}