Compute outline mid-line in way that works with negative start-values

Compute outline mid-line in way that works with negative start-values

Since the expression (x1+x2)/2 will always "round" towards zero, if one
of the points (usually x1) is negative, the wrong "mid-line" will be
computed, and as a result part of the outline will shift right/down.
Instead compute the mid-line as x1+(x2-x1)/2 (or x1+thickness/2) i.e
perform the division on something that is known to be non-negative and
then offset from the starting point. (Similar reasoning for the other
dimension.)

BUG=529010
Committed: https://crrev.com/c6cb6e422ed0508bf1a283f31175bbd65e098178
git-svn-id: svn://svn.chromium.org/blink/trunk@202205 bbb929c8-8fbe-4397-9dbb-9b2b20218538

[fs, 2015-09-14 14:39:52]

fs@opera.com changed reviewers:
+ fmalita@chromium.org

[f(malita), 2015-09-14 15:29:17]

LGTM

[fs, 2015-09-14 15:45:31]

The CQ bit was checked by fs@opera.com

[commit-bot: I haz the power, 2015-09-14 15:45:39]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1343773002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1343773002/1

[commit-bot: I haz the power, 2015-09-14 16:21:53]

Committed patchset #1 (id:1) as
https://src.chromium.org/viewvc/blink?view=rev&revision=202205

[commit-bot: I haz the power, 2015-09-23 12:32:51]

Patchset 1 (id:??) landed as
https://crrev.com/c6cb6e422ed0508bf1a283f31175bbd65e098178