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Currently, the bookmark bar uses the view hierarchy as its "model":
bookmark buttons are force ordered to the child view index that
matches their order in the model node (so that child_at(i) is the
ith button), and GetBookmarkButtonCount() is implemented as:

`return child_count() - 6;`

This is fragile, and less than ideal for keyboard traversal.

This change tracks the buttons separately in a vector, and maintains
proper focus traversal.

This is setup for Cocoa-style button dragging (as well as a fix
for issue 841785).

Bug: 712248, 841785
Change-Id: I236e34503d021ff0f27974f731f6abcb4d62f829
Reviewed-on: https://chromium-review.googlesource.com/1177908
Commit-Queue: Leonard Grey <lgrey@chromium.org>
Reviewed-by: Elly Fong-Jones <ellyjones@chromium.org>
Cr-Commit-Position: refs/heads/master@{#584117}