Revert "Remove Shadow DOM V0, Custom Element V0 and HTML Imports"

This reverts commit ad0f43e7.

Reason for revert: breaking youtube, crbug/937702

Original change's description:
> Remove Shadow DOM V0, Custom Element V0 and HTML Imports
>
> This patch removes Shadow DOM V0, Custom Element V0 (CEv0) and HTML Imports.
> Intent to Deprecate and Remove: http://bit.ly/2RGHe3x
>
> Bug: 911943
> Change-Id: I1baa906e019e0f59331c1fb590eccbc38a482fec
> Reviewed-on: https://chromium-review.googlesource.com/c/1481130
> Reviewed-by: Hayato Ito <hayato@chromium.org>
> Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
> Reviewed-by: Kent Tamura <tkent@chromium.org>
> Commit-Queue: Yoichi Osato <yoichio@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#636728}

TBR=kinuko@chromium.org,hayato@chromium.org,tkent@chromium.org,yoichio@chromium.org

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: 911943, 937702
Change-Id: I4aa4e34f3846ee8b5c2e19b29905243e0db5319a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1501935
Commit-Queue: Michael Moss <mmoss@chromium.org>
Reviewed-by: Michael Moss <mmoss@chromium.org>
Reviewed-by: Yoichi Osato <yoichio@chromium.org>
Cr-Commit-Position: refs/heads/master@{#637571}

third_party/blink/renderer/platform/runtime_enabled_features.json5

@@ -384,7 +384,7 @@
     {
       name: "CustomElementsV0",
       origin_trial_feature_name: "WebComponentsV0",
-      status: "test",
+      status: "stable",
     },
     {
       name: "CustomUserTiming",
@@ -613,7 +613,7 @@
     {
       name: "HTMLImports",
       origin_trial_feature_name: "WebComponentsV0",
-      status: "test",
+      status: "stable",
     },
     // Allow import only chrome internal resources.
     {
@@ -1266,7 +1266,7 @@
     {
       name: "ShadowDOMV0",
       origin_trial_feature_name: "WebComponentsV0",
-      status: "test",
+      status: "stable",
     },
     {
       name: "ShadowPiercingDescendantCombinator",

third_party/blink/web_tests/virtual/stable/webexposed/custom-elements-expected.txt

-CONSOLE WARNING: :unresolved pseudo selector is deprecated and will be removed in M73, around March 2019. Please use :not(:defined) instead. See https://www.chromestatus.com/features/4642138092470272 for more details.
-Tests basic web-exposure of Custom Elements
-
-On success, you will see a series of "PASS" messages, followed by "TEST COMPLETE".
-
-Document.prototype.hasOwnProperty("registerElement")
-false
-
-
-typeof Document.prototype.registerElement
-undefined
-
-
-document.createElement("x-a") instanceof HTMLUnknownElement
-false
-
-
-document.querySelector(":unresolved")
-[object HTMLDivElement]
-
-
-span.getAttribute("is")
-type-extension
-
-
-PASS window.customElements.define("a-a", A, {extends: "b"}) did not throw exception.
-PASS successfullyParsed is true
-
-TEST COMPLETE
-

third_party/blink/web_tests/virtual/stable/webexposed/element-instance-property-listing-expected.txt

@@ -53,6 +53,7 @@ namespace http://www.w3.org/1999/xhtml
     property computedStyleMap
     property contains
     property contentEditable
+    property createShadowRoot
     property dataset
     property dir
     property dispatchEvent
@@ -67,6 +68,7 @@ namespace http://www.w3.org/1999/xhtml
     property getAttributeNodeNS
     property getBoundingClientRect
     property getClientRects
+    property getDestinationInsertionPoints
     property getElementsByClassName
     property getElementsByTagName
     property getElementsByTagNameNS
@@ -689,6 +691,7 @@ html element link
     property hreflang
     property imageSizes
     property imageSrcset
+    property import
     property integrity
     property media
     property referrerPolicy
@@ -1153,6 +1156,7 @@ namespace http://www.w3.org/2000/svg
     property compareDocumentPosition
     property computedStyleMap
     property contains
+    property createShadowRoot
     property dataset
     property dispatchEvent
     property firstChild
@@ -1165,6 +1169,7 @@ namespace http://www.w3.org/2000/svg
     property getAttributeNodeNS
     property getBoundingClientRect
     property getClientRects
+    property getDestinationInsertionPoints
     property getElementsByClassName
     property getElementsByTagName
     property getElementsByTagNameNS

third_party/blink/web_tests/virtual/stable/webexposed/global-interface-listing-expected.txt

@@ -1365,6 +1365,7 @@ interface Document : Node
     method queryCommandValue
     method querySelector
     method querySelectorAll
+    method registerElement
     method releaseEvents
     method webkitCancelFullScreen
     method webkitExitFullscreen
@@ -1565,6 +1566,7 @@ interface Element : Node
     method closest
     method computedStyleMap
     method constructor
+    method createShadowRoot
     method getAttribute
     method getAttributeNS
     method getAttributeNames
@@ -1572,6 +1574,7 @@ interface Element : Node
     method getAttributeNodeNS
     method getBoundingClientRect
     method getClientRects
+    method getDestinationInsertionPoints
     method getElementsByClassName
     method getElementsByTagName
     method getElementsByTagNameNS
@@ -2688,6 +2691,7 @@ interface HTMLLinkElement : HTMLElement
     getter hreflang
     getter imageSizes
     getter imageSrcset
+    getter import
     getter integrity
     getter media
     getter referrerPolicy
@@ -6580,6 +6584,7 @@ interface Text : CharacterData
     getter assignedSlot
     getter wholeText
     method constructor
+    method getDestinationInsertionPoints
     method splitText
 interface TextDecoder
     attribute @@toStringTag