[PE] Allow transient PaintController to abort without commit

This disables DCHECK(new_display_item_list_.IsEmpty()) for transient
paint controllers in the destructor. We could require the client of
a transient controller to always commit and finish cycle, but allowing
to abort seems simpler and harmless.

Bug: 904971
Change-Id: If1bd15b55cbad0e1871eaed4a8d30b78e8a54d86
Reviewed-on: https://chromium-review.googlesource.com/c/1336953Reviewed-by: Chris Harrelson <chrishtr@chromium.org>
Commit-Queue: Xianzhu Wang <wangxianzhu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#608615}

third_party/blink/renderer/platform/graphics/paint/paint_controller.cc

@@ -25,8 +25,9 @@ PaintController::PaintController(Usage usage)
 }
 
 PaintController::~PaintController() {
-  // New display items should be committed before PaintController is destroyed.
-  DCHECK(new_display_item_list_.IsEmpty());
+  // New display items should be committed before PaintController is destroyed,
+  // except for transient paint controllers.
+  DCHECK(usage_ == kTransient || new_display_item_list_.IsEmpty());
 }
 
 bool PaintController::UseCachedItemIfPossible(const DisplayItemClient& client,

third_party/blink/renderer/platform/graphics/paint/paint_controller_test.cc

@@ -1556,6 +1556,17 @@ TEST_P(PaintControllerTest, InsertValidItemInFront) {
   EXPECT_TRUE(fourth.IsValid());
 }
 
+TEST_P(PaintControllerTest, TransientPaintControllerIncompleteCycle) {
+  auto paint_controller = PaintController::Create(PaintController::kTransient);
+  GraphicsContext context(*paint_controller);
+  FakeDisplayItemClient client("client", LayoutRect(100, 100, 50, 50));
+  InitRootChunk(*paint_controller);
+  DrawRect(context, client, kBackgroundType, FloatRect(100, 100, 50, 50));
+  // The client of a transient paint controller can abort without
+  // CommintNewDisplayItems() and FinishCycle(). This should not crash.
+  paint_controller = nullptr;
+}
+
 // Death tests don't work properly on Android.
 #if defined(GTEST_HAS_DEATH_TEST) && !defined(OS_ANDROID)