[SP] Always anti-alias borders

[SP] Always anti-alias borders

When slimming paint is enabled, skip the CTM dependent heuristic and
always anti-alias borders.

This relies on heavy use of overdraw, which is now applicable in most
cases thanks to the recent BoxBorderPainter changes.

BUG=474265
R=chrishtr@chromium.org,fs@opera.com

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=196779

[f(malita), 2015-06-09 14:13:32]

After all the BoxBorderPainter massaging, we can finally flip the switch on AA
borders without introducing visible corner seams!

The layout test diffs are due to changes in miter anti-aliasing:
https://storage.googleapis.com/chromium-layout-test-archives/linux_blink_rel/...

Unfortunately, since SP vs. non-SP can now render slightly different (mainly
miter AA), we also have four SP ref-tests failures.  I've suppressed them for
now - we can either turn them into pixel tests later or wait for SP to stick.

No perf impact measured locally.

[fs, 2015-06-09 15:52:08]

LGTM, especially the re -> er bit! =D

[chrishtr, 2015-06-09 16:30:52]

The CQ bit was checked by chrishtr@chromium.org

[chrishtr, 2015-06-09 16:30:53]

lgtm

https://codereview.chromium.org/1167293004/diff/40001/Source/core/paint/BoxBo...
File Source/core/paint/BoxBorderPainter.h (left):

https://codereview.chromium.org/1167293004/diff/40001/Source/core/paint/BoxBo...
Source/core/paint/BoxBorderPainter.h:31: enum MitreType {
American English FTW!

[commit-bot: I haz the power, 2015-06-09 16:31:10]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1167293004/40001

[commit-bot: I haz the power, 2015-06-09 16:35:06]

Committed patchset #3 (id:40001) as
https://src.chromium.org/viewvc/blink?view=rev&revision=196779