Fix flaky paint property test that relied on static state

PaintPropertyNodeTest.InitialStateAndReset had an expectation that the
initial root paint property state was changed. This could fail if the
test was run after another test that modified the root node's state.
This patch initializes the root node state to be unchanged.

Bug: 879017
Cq-Include-Trybots: luci.chromium.try:linux_layout_tests_slimming_paint_v2;master.tryserver.blink:linux_trusty_blink_rel
Change-Id: If30731871ac8bffb66f8b3a387510e7e2d6fea56
Reviewed-on: https://chromium-review.googlesource.com/1196824Reviewed-by: Xianzhu Wang <wangxianzhu@chromium.org>
Commit-Queue: Philip Rogers <pdr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#587733}

third_party/blink/renderer/platform/graphics/paint/paint_property_node.h

@@ -91,7 +91,8 @@ class PaintPropertyNode : public RefCounted<NodeType> {
 #endif
 
  protected:
-  PaintPropertyNode(const NodeType* parent) : parent_(parent) {}
+  PaintPropertyNode(const NodeType* parent)
+      : parent_(parent), changed_(!!parent) {}
 
   bool SetParent(const NodeType* parent) {
     DCHECK(!IsRoot());
@@ -104,14 +105,17 @@ class PaintPropertyNode : public RefCounted<NodeType> {
     return true;
   }
 
-  void SetChanged() { changed_ = true; }
+  void SetChanged() {
+    DCHECK(!IsRoot());
+    changed_ = true;
+  }
   bool NodeChanged() const { return changed_; }
 
  private:
   friend class PaintPropertyNodeTest;
 
   scoped_refptr<const NodeType> parent_;
-  mutable bool changed_ = true;
+  mutable bool changed_;
 
 #if DCHECK_IS_ON()
   String debug_name_;

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

@@ -84,7 +84,7 @@ class PaintPropertyNodeTest : public testing::Test {
 
   template <typename NodeType>
   void ExpectInitialState(const Tree<NodeType>& tree) {
-    EXPECT_TRUE(tree.root->NodeChanged());
+    EXPECT_FALSE(tree.root->NodeChanged());
     EXPECT_TRUE(tree.ancestor->NodeChanged());
     EXPECT_TRUE(tree.child1->NodeChanged());
     EXPECT_TRUE(tree.child2->NodeChanged());