Add "HTMLImports" runtime flag for Blink

This patch introduces the flag for the HTML Imports deprecation.

Use with --disable-blink-features=HTMLImports
flag on content_shell or chrome.

Bug: 766694
Change-Id: I9c0836f3fc05429d34062dc0fcc2eda209cc1cf0
Reviewed-on: https://chromium-review.googlesource.com/998455Reviewed-by: Yoichi Osato <yoichio@chromium.org>
Reviewed-by: Takayoshi Kochi <kochi@chromium.org>
Reviewed-by: Kent Tamura <tkent@chromium.org>
Commit-Queue: Kent Tamura <tkent@chromium.org>
Cr-Commit-Position: refs/heads/master@{#549394}

third_party/blink/renderer/core/html/html_link_element.cc

@@ -144,7 +144,8 @@ LinkResource* HTMLLinkElement::LinkResourceToProcess() {
   }
 
   if (!link_) {
-    if (rel_attribute_.IsImport()) {
+    if (rel_attribute_.IsImport() &&
+        RuntimeEnabledFeatures::HTMLImportsEnabled()) {
       link_ = LinkImport::Create(this);
     } else if (rel_attribute_.IsManifest()) {
       link_ = LinkManifest::Create(this);

third_party/blink/renderer/core/html/html_link_element.idl

@@ -48,7 +48,7 @@ interface HTMLLinkElement : HTMLElement {
 
     // HTML Imports
     // https://w3c.github.io/webcomponents/spec/imports/#interface-import
-    readonly attribute Document? import;
+    [RuntimeEnabled=HTMLImports] readonly attribute Document? import;
 
     // Subresource Integrity
     // https://w3c.github.io/webappsec-subresource-integrity/#HTMLLinkElement

third_party/blink/renderer/platform/runtime_enabled_features.json5

@@ -538,6 +538,11 @@
     {
       name: "HeapIncrementalMarking",
     },
+    // https://crbug.com/766694 for testing disabling the feature.
+    {
+      name: "HTMLImports",
+      status: "stable",
+    },
     // https://crbug.com/523952 for testing disabling the feature.
     {
       name: "HTMLImportsStyleApplication",