Implement basic declarative Shadow DOM

This CL adds the DeclarativeShadowDOM runtime enabled feature, and
implements a basic approach. There are no tests yet, as this is an
evolving spec/approach. But this allows basic testing of attaching
the shadow root upon the closing <template> tag.

See the explainer [1], the issue discussion [2], the intent to
prototype [3], and the chromestatus entry [4].

To test this feature:
  --enable-blink-features=DeclarativeShadowDOM

and then:
  <div>
    <template shadowroot=open>
      <div></div>
      <slot></slot>
    </template>
    <span>
  </div>

[1] https://github.com/mfreed7/declarative-shadow-dom/blob/master/README.md
[2] https://github.com/whatwg/dom/issues/831
[3] https://groups.google.com/a/chromium.org/forum/#!topic/blink-dev/nJDc-1s3R9U
[4] https://chromestatus.com/feature/5191745052606464

Bug: 1042130

Change-Id: I5057c932a89fc401735d024348799d30c97e959e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2018336
Auto-Submit: Mason Freed <masonfreed@chromium.org>
Reviewed-by: Kouhei Ueno <kouhei@chromium.org>
Commit-Queue: Mason Freed <masonfreed@chromium.org>
Cr-Commit-Position: refs/heads/master@{#741538}

third_party/blink/renderer/core/dom/element.cc

@@ -3528,6 +3528,21 @@ ShadowRoot& Element::CreateAndAttachShadowRoot(ShadowRootType type) {
   return *shadow_root;
 }
 
+void Element::AttachDeclarativeShadowRoot(HTMLTemplateElement* template_element,
+                                          ShadowRootType type) {
+  DCHECK(template_element);
+  DCHECK(type == ShadowRootType::kOpen || type == ShadowRootType::kClosed);
+  if (!CanAttachShadowRoot()) {
+    // TODO(masonfreed): Eventually, this should be a DOMException.
+    LOG(ERROR) << "Invalid shadow root host element";
+    return;
+  }
+  ShadowRoot* shadow_root = &AttachShadowRootInternal(
+      type, /*delegates_focus=*/false, /*manual_slotting=*/false);
+  shadow_root->appendChild(template_element->content());
+  template_element->remove();
+}
+
 ShadowRoot* Element::GetShadowRoot() const {
   return HasRareData() ? GetElementRareData()->GetShadowRoot() : nullptr;
 }

third_party/blink/renderer/core/dom/element.h

@@ -65,6 +65,7 @@ class ElementRareData;
 class ExceptionState;
 class FloatQuad;
 class FloatSize;
+class HTMLTemplateElement;
 class Image;
 class InputDeviceCapabilities;
 class Locale;
@@ -574,6 +575,9 @@ class CORE_EXPORT Element : public ContainerNode, public Animatable {
   ShadowRoot* createShadowRoot(ExceptionState&);
   ShadowRoot* attachShadow(const ShadowRootInit*, ExceptionState&);
 
+  void AttachDeclarativeShadowRoot(HTMLTemplateElement* template_element,
+                                   ShadowRootType type);
+
   ShadowRoot& CreateV0ShadowRootForTesting() {
     return CreateShadowRootInternal();
   }

third_party/blink/renderer/core/html/html_attribute_names.json5

@@ -295,6 +295,7 @@
     "scrolling",
     "select",
     "selected",
+    "shadowroot",
     "shape",
     "size",
     "sizes",

third_party/blink/renderer/core/html/parser/html_tree_builder.cc

@@ -32,6 +32,7 @@
 #include "third_party/blink/renderer/core/dom/document.h"
 #include "third_party/blink/renderer/core/dom/document_fragment.h"
 #include "third_party/blink/renderer/core/dom/element_traversal.h"
+#include "third_party/blink/renderer/core/dom/shadow_root.h"
 #include "third_party/blink/renderer/core/frame/web_feature.h"
 #include "third_party/blink/renderer/core/html/forms/html_form_control_element.h"
 #include "third_party/blink/renderer/core/html/forms/html_form_element.h"
@@ -909,10 +910,38 @@ bool HTMLTreeBuilder::ProcessTemplateEndTag(AtomicHTMLToken* token) {
   tree_.GenerateImpliedEndTags();
   if (!tree_.CurrentStackItem()->HasTagName(html_names::kTemplateTag))
     ParseError(token);
-  tree_.OpenElements()->PopUntilPopped(html_names::kTemplateTag);
+  tree_.OpenElements()->PopUntil(html_names::kTemplateTag.LocalName());
+  HTMLStackItem* template_stack_item =
+      tree_.OpenElements()->TopRecord()->StackItem();
+  tree_.OpenElements()->Pop();
+  HTMLStackItem* shadow_host_stack_item =
+      tree_.OpenElements()->TopRecord()->StackItem();
   tree_.ActiveFormattingElements()->ClearToLastMarker();
   template_insertion_modes_.pop_back();
   ResetInsertionModeAppropriately();
+  if (RuntimeEnabledFeatures::DeclarativeShadowDOMEnabled() &&
+      template_stack_item && template_stack_item->IsElementNode() &&
+      shadow_host_stack_item && shadow_host_stack_item->IsElementNode()) {
+    // TODO(masonfreed): Add a use counter here.
+    HTMLTemplateElement* template_element =
+        DynamicTo<HTMLTemplateElement>(template_stack_item->GetElement());
+    DCHECK(template_element);
+    Element* shadow_host = shadow_host_stack_item->GetElement();
+    DCHECK(shadow_host);
+    const auto& shadow_mode =
+        template_element->FastGetAttribute(html_names::kShadowrootAttr);
+    if (shadow_mode != g_null_atom) {
+      bool is_open = EqualIgnoringASCIICase(shadow_mode, "open");
+      if (is_open || EqualIgnoringASCIICase(shadow_mode, "closed")) {
+        shadow_host->AttachDeclarativeShadowRoot(
+            template_element,
+            is_open ? ShadowRootType::kOpen : ShadowRootType::kClosed);
+      } else {
+        // TODO(masonfreed): eventually, console warning here.
+        LOG(ERROR) << "Invalid shadowroot value " << shadow_mode;
+      }
+    }
+  }
   return true;
 }
 

third_party/blink/renderer/platform/runtime_enabled_features.json5

@@ -571,6 +571,10 @@
       name: "Database",
       status: "stable",
     },
+    {
+      name: "DeclarativeShadowDOM",
+      status: "test",
+    },
     {
       name: "DecodeJpeg420ImagesToYUV",
       status: "test",