[SPv175+] Will-change without layer should not have direct compositing reason

For <svg style="will-change: left"></svg>,
previously we created a transform node just to store the direct
compositing reason CompositingReason::kWillChangeCompositingHint,
which broke the assumption that only objects having a PaintLayer
can be composited.

Bug: 825090
Cq-Include-Trybots: master.tryserver.blink:linux_trusty_blink_rel;master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2
Change-Id: I6eddb3b025ea82c08b38dc4ae0c0efc1048f6854
Reviewed-on: https://chromium-review.googlesource.com/978504Reviewed-by: Chris Harrelson <chrishtr@chromium.org>
Commit-Queue: Xianzhu Wang <wangxianzhu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#545571}

third_party/WebKit/Source/core/paint/PaintPropertyTreeBuilder.cpp

@@ -580,6 +580,9 @@ void FragmentPaintPropertyTreeBuilder::UpdateTransformForNonRootSVG() {
 }
 
 static CompositingReasons CompositingReasonsForTransform(const LayoutBox& box) {
+  if (!box.HasLayer())
+    return CompositingReason::kNone;
+
   const ComputedStyle& style = box.StyleRef();
   CompositingReasons compositing_reasons = CompositingReason::kNone;
   if (CompositingReasonFinder::RequiresCompositingForTransform(box))

third_party/WebKit/Source/core/paint/PaintPropertyTreeBuilderTest.cpp

@@ -5360,4 +5360,10 @@ TEST_P(PaintPropertyTreeBuilderTest, IframeDoesNotRequireCompositedScrolling) {
                    ->HasDirectCompositingReasons());
 }
 
+TEST_P(PaintPropertyTreeBuilderTest,
+       NoTransformPropertyForWillChangeWithoutLayer) {
+  SetBodyInnerHTML("<svg id='target' style='will-change: left'></svg>");
+  EXPECT_EQ(nullptr, PaintPropertiesForElement("target")->Transform());
+}
+
 }  // namespace blink