Fix a FORWARD_NULL defect reported by Coverity....

[Replaced by http://codereview.chromium.org/8827007 ]

Fix a FORWARD_NULL defect reported by Coverity.

We never construct a ScrollView object with a null horiz_sb_
or vert_sb_ member, and we already dereference horiz_sb_ and
vert_sb_ without null checking in many places (including the
destructor), so all the null checks for horiz_sb_ and vert_sb_
are unnecessary.

R=ben
BUG=http://crbug.com/17101
TEST=none

[Ben Goodger (Google), 2011-11-30 16:26:13]

lgtm

[wtc, 2011-12-06 22:10:05]

I lost the source tree that has this CL, so I created a new
CL http://codereview.chromium.org/8827007 to check this change
in.