Enable slimming paint by default (for trybots only)

sp/gen-auto-rebaseline

+#!/bin/sh
+# Collect recent auto-rebaselines of base tests and generate NeedsRebaseline
+# for slimming paint.
+cd `dirname $0`/..
+git remote update
+revisions=$(git log $(cat sp/last-auto-rebaseline)..origin/master --reverse --grep Auto-rebaseline --format=oneline|awk '{print $1}') 
+declare -A tests
+for i in $revisions; do
+  for t in `git show $i LayoutTests/TestExpectations | grep "^-[cB].*NeedsRebaseline" |
+      sed 's/\[[^]]*\]//g' | awk '{print $2}'`; do
+    if [[ $t == virtual/slimmingpaint/* ]]; then
+      unset tests[$t]
+    elif [[ $t != virtual/* ]]; then
+      tests[virtual/slimmingpaint/$t]=
+    fi
+  done
+  last=$i
+done
+for i in ${!tests[@]}; do
+  echo "Bug(wangxianzhu) $i [ NeedsRebaseline ]"
+done | sort
+if [ "$last" == "" ]; then
+  last=`git log origin/master~1..origin/master | awk '{print $1}'`
+fi
+echo $last >sp/last-auto-rebaseline