Use plus/minus 0.5 instead of 1 when calculating zoomed length.

Plus/minus 0.5 to the truncated value is more reasonable than
directly adding 1, because 0.5 is the average of the fraction
value.

This patch fixes Acid3 test 99/100 when zoom is 125%, and also
improves overall image zooming.

This patch requires a content shell change to set zoom factor
in script (CL 253493006).

BUG=360713

Review URL: https://codereview.chromium.org/251063002

git-svn-id: svn://svn.chromium.org/blink/trunk@172977 bbb929c8-8fbe-4397-9dbb-9b2b20218538