Add stable id to PaintImage.

cc/paint/discardable_image_map_unittest.cc

Index: cc/paint/discardable_image_map_unittest.cc
diff --git a/cc/paint/discardable_image_map_unittest.cc b/cc/paint/discardable_image_map_unittest.cc
index 015e539141cae96a76a2d6df1e93cb68f314d47e..02124e1af85c47a46ee4701d7f7a44956db041de 100644
--- a/cc/paint/discardable_image_map_unittest.cc
+++ b/cc/paint/discardable_image_map_unittest.cc
@@ -44,10 +44,10 @@ sk_sp<PaintRecord> CreateRecording(const sk_sp<SkImage>& discardable_image,
   PaintRecorder recorder;
   PaintCanvas* canvas =
       recorder.beginRecording(visible_rect.width(), visible_rect.height());
-  canvas->drawImage(
-      PaintImage(discardable_image, PaintImage::AnimationType::STATIC,
-                 PaintImage::CompletionState::DONE),
-      0, 0, nullptr);
+  canvas->drawImage(PaintImage(PaintImage::GetNextId(), discardable_image,
+                               PaintImage::AnimationType::STATIC,
+                               PaintImage::CompletionState::DONE),
+                    0, 0, nullptr);
   sk_sp<PaintRecord> record = recorder.finishRecordingAsPicture();
   return record;
 }
@@ -678,7 +678,8 @@ TEST_F(DiscardableImageMapTest, GathersDiscardableImagesFromNestedOps) {
   sk_sp<SkImage> discardable_image =
       CreateDiscardableImage(gfx::Size(100, 100));
   internal_record->push<DrawImageOp>(
-      PaintImage(discardable_image, PaintImage::AnimationType::STATIC,
+      PaintImage(PaintImage::GetNextId(), discardable_image,
+                 PaintImage::AnimationType::STATIC,
                  PaintImage::CompletionState::DONE),
       0.f, 0.f, nullptr);
 
@@ -686,7 +687,8 @@ TEST_F(DiscardableImageMapTest, GathersDiscardableImagesFromNestedOps) {
   sk_sp<SkImage> discardable_image2 =
       CreateDiscardableImage(gfx::Size(100, 100));
   list_record->push<DrawImageOp>(
-      PaintImage(discardable_image2, PaintImage::AnimationType::STATIC,
+      PaintImage(PaintImage::GetNextId(), discardable_image2,
+                 PaintImage::AnimationType::STATIC,
                  PaintImage::CompletionState::DONE),
       100.f, 100.f, nullptr);
   scoped_refptr<DisplayItemList> display_list = new DisplayItemList;