pdfs from Cole.
BUG=42026

TEST=\
1) Small folders --> no arrows.
2) Big folders --> arrow at bottom initially.
3) Move browser to bottom of screen so a small folder falls off the bottom and has an arrow.
Open it and gently use scroll wheel to scroll.  Make sure transition to "no arrow" looks good.
Close and reopen.  Scroll super-fast.  Make sure it ends up in the same nice place.
4) Open big big folder.  Scroll so top goes offscreen so you now have 2 scroll arrows.
Use scroll wheel to gently go up and down (arrow hides and shows).  Make sure transitions OK.
Scroll all the way so bottom arrow disappears.  Gently up and down; watch for transitions.
Now FAST up and down.  Make sure destination looks OK.

BookmarkBarFolderWindow.xib change: BookmarkBarFolderWindowScrollView border turned off.

Review URL: http://codereview.chromium.org/1813003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@46114 0039d316-1c4b-4281-b951-d872f2087c98