[BlinkGenPropertyTrees] Do not clear paint controller in SPV2 mode

This patch is a followup to https://crrev.com/601428 and stops clearing
the paint controller in SPV2 mode. This prevents flashing on all pages
when running with SPV2+BGPT and will unblock making BGPT experimental
(all SPV2 layout tests fail without this change):
  https://chromium-review.googlesource.com/c/chromium/src/+/1294204

Bug: 836902
Change-Id: I80bea03bc3d06c5fe6761734babdf6517813e7bb
Reviewed-on: https://chromium-review.googlesource.com/c/1297060Reviewed-by: Xianzhu Wang <wangxianzhu@chromium.org>
Commit-Queue: Philip Rogers <pdr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#602128}

third_party/blink/renderer/core/frame/local_frame_view.cc

@@ -2622,8 +2622,10 @@ void LocalFrameView::RunPaintLifecyclePhase() {
       // lifecycle state can be freed (such as raster invalidations).
       paint_controller_->FinishCycle();
       // PaintController for BlinkGenPropertyTrees is transient.
-      if (RuntimeEnabledFeatures::BlinkGenPropertyTreesEnabled())
+      if (RuntimeEnabledFeatures::BlinkGenPropertyTreesEnabled() &&
+          !RuntimeEnabledFeatures::SlimmingPaintV2Enabled()) {
         paint_controller_ = nullptr;
+      }
     }
   }
 }