turning on slimming paint by default

[Walter Korman]

Hi Paint Team,

From discussion today, below is our updated plan with particular focus on how to deal with the mass of LayoutTest updates that will be needed in non-virtual-test-suite land.

1. Modify layout test runner script to turn off slimming paint by default, passing --disable-slimming-paint (to be actually introduced in code in next step). Preparing change shortly.

2. Commit http://crrev.com/1169193003 to change Chromium-side slimming paint flag to '--disable-slimming-paint' from '--enable-slimming-paint', forcing slimming paint on by default, but with a way to disable it, and already in place Blink-side per #1, thus existing LayoutTests should continue w/ existing non-SP expectations.

3. Follow approach impl-side-painting used by incrementally turning on slimming paint for Blink-side LayoutTests on a per-directory basis, ala http://crrev.com/813473005/ thus giving us a chance to field rebaselines and code reviews for LayoutTests in a reasoned manner over time. We need to make sure having both '--disable-slimming-paint' and '--enable-slimming-paint' flags results in slimming paint actually being enabled. Note we'll have removed the Chromium-side '--enable-slimming-paint' flag at this point per #2, but we're keeping it named as-is Blink-side, and I'm presuming we can still use --enable-blink-features=slimmingPaintEnabled.

[Walter Korman]

Edit/clarification of #3: we're not removing '--enable-slimming-paint' flag in #2, rather we've just added an additional '--disable-slimming-paint' flag, and enable takes precedence over disable. So we don't need to use --enable-blink-features=slimmingPaintEnabled.