Add config for Linux layout test try bot with --enable-slimming-paint-v2.

masters/master.tryserver.chromium.linux/master.cfg

Index: masters/master.tryserver.chromium.linux/master.cfg
diff --git a/masters/master.tryserver.chromium.linux/master.cfg b/masters/master.tryserver.chromium.linux/master.cfg
index f5213bf9e0f1ba5b920d6ce045d6c4082c8129f0..baf428698d7cca204cb3a294f568b18daa04712d 100644
--- a/masters/master.tryserver.chromium.linux/master.cfg
+++ b/masters/master.tryserver.chromium.linux/master.cfg
@@ -275,6 +275,12 @@ chromium_builders.append({
   'slavebuilddir': 'linux',
 })
 
+# Builder specifically to run layout tests with a particular flag (see crbug.com/622865).
+chromium_builders.append({
+  'name': 'linux_layout_tests_slimming_paint_v2',
+  'factory': m_remote_run('chromium_trybot'),
+})
+
 # Presubmit bots
 def presubmit(name, slavebuilddir=None, extra_props=None):
   properties = {'repo_name': name}