GTK: Fix minor slide animation bugs.
GTK: Fix minor slide animation bugs.

Following r27934, we might hit the code path for initial positioning *after* the AnimationProgressed positioning, causing the child widget not to show.

BUG=23902
TEST=when you switch away from a tab that has an infobar, then switch back, you can see the infobar.

Review URL: http://codereview.chromium.org/258038

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@28214 0039d316-1c4b-4281-b951-d872f2087c98

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