Simplify LayoutSVGModelObject/Block blend-mode style-change handling

Use StyleDifference::BlendModeChanged() to check for changes. The
initial isolation notification will be handled by the AddChild() on the
container.

Switch the call to IsBlendingAllowed() to a more targeted call (or no
call at all).

drop that from LayoutSVGModelObject.

LayoutObject: :SetStyle() already marks paint properties for updating, so
Change-Id: I2d43d6b314509db0881003809375aaf0d8f3a292
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2463829Reviewed-by: Stephen Chenney <schenney@chromium.org>
Commit-Queue: Fredrik Söderquist <fs@opera.com>
Cr-Commit-Position: refs/heads/master@{#816475}

third_party/blink/renderer/core/layout/svg/layout_svg_block.cc

@@ -126,14 +126,11 @@ void LayoutSVGBlock::StyleDidChange(StyleDifference diff,
       SetNeedsTransformUpdate();
   }
 
-  if (IsBlendingAllowed()) {
-    bool has_blend_mode_changed =
-        (old_style && old_style->HasBlendMode()) == !StyleRef().HasBlendMode();
-    if (Parent() && has_blend_mode_changed) {
-      Parent()->DescendantIsolationRequirementsChanged(
-          StyleRef().HasBlendMode() ? kDescendantIsolationRequired
-                                    : kDescendantIsolationNeedsUpdate);
-    }
+  if (diff.BlendModeChanged() && Parent()) {
+    DCHECK(IsBlendingAllowed());
+    Parent()->DescendantIsolationRequirementsChanged(
+        StyleRef().HasBlendMode() ? kDescendantIsolationRequired
+                                  : kDescendantIsolationNeedsUpdate);
   }
 
   if (diff.CompositingReasonsChanged())

third_party/blink/renderer/core/layout/svg/layout_svg_model_object.cc

@@ -154,17 +154,11 @@ void LayoutSVGModelObject::StyleDidChange(StyleDifference diff,
       SetNeedsTransformUpdate();
   }
 
-  if (IsBlendingAllowed()) {
-    bool has_blend_mode_changed =
-        (old_style && old_style->HasBlendMode()) == !StyleRef().HasBlendMode();
-    if (Parent() && has_blend_mode_changed) {
-      Parent()->DescendantIsolationRequirementsChanged(
-          StyleRef().HasBlendMode() ? kDescendantIsolationRequired
-                                    : kDescendantIsolationNeedsUpdate);
-    }
-
-    if (has_blend_mode_changed)
-      SetNeedsPaintPropertyUpdate();
+  if (diff.BlendModeChanged() && Parent() && !IsSVGHiddenContainer()) {
+    DCHECK(IsBlendingAllowed());
+    Parent()->DescendantIsolationRequirementsChanged(
+        StyleRef().HasBlendMode() ? kDescendantIsolationRequired
+                                  : kDescendantIsolationNeedsUpdate);
   }
 
   if (diff.CompositingReasonsChanged())