[SP] Enable Slimming Paint by default in Chromium and Blink.

[SP] Enable Slimming Paint by default in Chromium and Blink.

Re-do of http://crrev.com/1169193003

Add a --disable-slimming-paint flag and make slimming paint
on by default with no flags specified. --enable-slimming-paint
takes precedence over --disable-slimming-paint if both are
specified.

Existence of both flags is required to deal gracefully with
Blink-side LayoutTests and a gradual migration. For more, see:

https://groups.google.com/a/chromium.org/d/msg/paint-dev/gtRRHn1yotk/f_8iJgHP130J
https://groups.google.com/a/chromium.org/d/msg/paint-dev/uVGvSuVilaw/uA3iVa7JpuUJ

BUG=497950
TBR=pdr,jam,mpearson,chrishtr,enne

Committed: https://crrev.com/260fd855786fb493ad780eb0a5a3681e50d0f941
Cr-Commit-Position: refs/heads/master@{#334686}

[wkorman, 2015-06-16 18:14:29]

wkorman@chromium.org changed reviewers:
+ chrishtr@chromium.org, pdr@chromium.org

[pdr., 2015-06-16 18:17:11]

TBR'd reviewers: This is the exact same change, but now with many fixes landed
that popped up the first time this was attempted.

LGTM

[wkorman, 2015-06-16 18:24:09]

The CQ bit was checked by wkorman@chromium.org

[commit-bot: I haz the power, 2015-06-16 18:25:16]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1174183003/20001

[chrishtr, 2015-06-16 18:29:38]

lgtm

[commit-bot: I haz the power, 2015-06-16 20:46:22]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-06-16 20:47:13]

Patchset 2 (id:??) landed as
https://crrev.com/260fd855786fb493ad780eb0a5a3681e50d0f941
Cr-Commit-Position: refs/heads/master@{#334686}

[wkorman, 2015-06-16 20:48:11]

Sheriff FYI -- I will be OOO ~2pm - 5pm today but on email periodically and
following, please contact pdr@ if I don't respond to any issues.