mac: Fix bookmark bar docking animation if the user has no bookmarks.
mac: Fix bookmark bar docking animation if the user has no bookmarks.

xib changes: Pin buttonView_, [buttonView_ noItemTextfield], and
[buttonView_ importBookmarksButton] to both top and bottom, so that they
stay centered when changing the height of the bookmarks bar during the
Instant Extended bookmark bar docking animation on the NTP.

BUG=225827
TEST=Open NTP with no bookmarks, hit cmd-shift-b, watch bookmarks bar animation.
Watch bookmarks bar animation. "For quick access, ..." text stays centered
and animates nicely instead of jumping around.

Review URL: https://codereview.chromium.org/13611002

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@192336 0039d316-1c4b-4281-b951-d872f2087c98

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