Revert of [SP] Turn off slimming paint by default.

Revert of [SP] Turn off slimming paint by default. (patchset #2 id:20001 of https://codereview.chromium.org/1186623003/)

Reason for revert:
crbug.com/501015 made it hard to anaylyze the regressions in crbug.com/501251. the issue causing crbug.com/501015 has been reverted, so it's time to turn slimming paint back on.

Original issue's description:
> [SP] Turn off slimming paint by default.
>
> Partial revert of http://crrev.com/1169193003 while we explore performance
> regressions.
>
> Leave the --disable-slimming-paint switch in existence, but restore
> prior logic around switches.
>
> BUG=497950, 501251
> TBR=jochen
>
> Committed: https://crrev.com/3908f356c8af559adb6941adf4df77432b8275c2
> Cr-Commit-Position: refs/heads/master@{#335386}

TBR=jochen@chromium.org,wkorman@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=497950, 501251
Committed: https://crrev.com/8db3efc9bb514d4e55a17fbd4d654c4ec4e72487
Cr-Commit-Position: refs/heads/master@{#335455}

[chrishtr, 2015-06-21 18:47:38]

Created Revert of [SP] Turn off slimming paint by default.

[commit-bot: I haz the power, 2015-06-21 18:47:49]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1200693002/1

[chrishtr, 2015-06-21 18:48:02]

The CQ bit was unchecked by chrishtr@chromium.org

[commit-bot: I haz the power, 2015-06-21 18:48:09]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-06-21 18:48:18]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-06-21 18:49:10]

Patchset 1 (id:??) landed as
https://crrev.com/8db3efc9bb514d4e55a17fbd4d654c4ec4e72487
Cr-Commit-Position: refs/heads/master@{#335455}

[wkorman, 2015-06-21 19:19:45]

lgtm