Fix show more button

Bug introduced in Change 296136, Show all button with new change
summary feature flag flips collapsed on .viewer, and without
feature flag collapsed is flipped on :host gr-editable-content.

This is fixed by setting css on .viewer[collapsed] only when
feature flag is enabled.

Change-Id: I407533bf951089e1ca801152986067278187b055
diff --git a/polygerrit-ui/app/elements/shared/gr-editable-content/gr-editable-content_html.ts b/polygerrit-ui/app/elements/shared/gr-editable-content/gr-editable-content_html.ts
index 6605394..73dc2d2 100644
--- a/polygerrit-ui/app/elements/shared/gr-editable-content/gr-editable-content_html.ts
+++ b/polygerrit-ui/app/elements/shared/gr-editable-content/gr-editable-content_html.ts
@@ -32,7 +32,7 @@
       padding: var(--spacing-m);
     }
     :host([collapsed]) .viewer,
-    .viewer[collapsed] {
+    .viewer.new-change-summary-true[collapsed] {
       max-height: var(--collapsed-max-height, 300px);
       overflow: hidden;
     }