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

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

Review URL: https://codereview.chromium.org/1169193003

Cr-Commit-Position: refs/heads/master@{#334188}
10 files changed