Remove HTMLImportsStyleApplication flag

This flag was on by default but only took effect if HTML Imports
was also enabled. The [1] commit disabled Custom Elements v0 and
Shadow DOM v0, but it also disabled the HTMLImportsStyleApplication
flag. This broke some sites (notably Youtube) that use HTML
Imports with imported style sheets. This CL fixes that breakage,
and removes this unnecessary flag.

[1] https://chromium.googlesource.com/chromium/src/+/f52dff0d3c5bc52ba608920f843162832f2da6b1

Bug: 1017069
Change-Id: I0f806635185932abab8d675fdb0814ede1fb8465
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1876744Reviewed-by: Philip Rogers <pdr@chromium.org>
Commit-Queue: Mason Freed <masonfreed@chromium.org>
Auto-Submit: Mason Freed <masonfreed@chromium.org>
Cr-Commit-Position: refs/heads/master@{#708777}

third_party/blink/renderer/core/css/style_engine.cc

@@ -446,9 +446,7 @@ void StyleEngine::MediaQueryAffectingValueChanged() {
 void StyleEngine::UpdateActiveStyleSheetsInImport(
     StyleEngine& master_engine,
     DocumentStyleSheetCollector& parent_collector) {
-  if (!RuntimeEnabledFeatures::HTMLImportsStyleApplicationEnabled())
-    return;
-
+  DCHECK(RuntimeEnabledFeatures::HTMLImportsEnabled(&GetDocument()));
   DCHECK(!IsMaster());
   HeapVector<Member<StyleSheet>> sheets_for_list;
   ImportedDocumentStyleSheetCollector subcollector(parent_collector,

third_party/blink/renderer/platform/runtime_enabled_features.json5

@@ -761,12 +761,6 @@
     {
       name: "HTMLImportsOnlyChrome",
     },
-    // TODO(937746): Web Components v0 is disabled by default, and will be
-    // removed after M87.
-    {
-      name: "HTMLImportsStyleApplication",
-      status: "test",
-    },
     {
       name: "IDBObserver",
       status: "experimental",