Initialize the buffer partition implicitly.

(This is a subset of previously reviewed
https://codereview.chromium.org/25644002/)

BUG=270531
TBR=tsepez@chromium.org

Review URL: https://codereview.chromium.org/26876003

git-svn-id: svn://svn.chromium.org/blink/trunk@159369 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Fix ScrollingCoordinatorChromiumTest.nonFastScrollableRegion.

This should have been part of https://codereview.chromium.org/26520003/,
sorry.

BUG=
R=ojan@chromium.org

Review URL: https://codereview.chromium.org/26851004

git-svn-id: svn://svn.chromium.org/blink/trunk@159370 bbb929c8-8fbe-4397-9dbb-9b2b20218538