Disable DrawingDisplayItem opaqueness detection for pre-CompositeAfterPaint

The result has no effect in pre-CompositeAfterPaint. Disable it to avoid
performance impact and reduce risk.

Bug: 1064712
Change-Id: I4f826c719c581b86b6ff2508b710508ef291e788
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2122618Reviewed-by: Philip Rogers <pdr@chromium.org>
Commit-Queue: Xianzhu Wang <wangxianzhu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#753787}

third_party/blink/renderer/platform/graphics/paint/drawing_display_item.h

@@ -37,6 +37,8 @@ class PLATFORM_EXPORT DrawingDisplayItem : public DisplayItem {
   bool Equals(const DisplayItem& other) const final;
 
   bool KnownToBeOpaque() const {
+    if (!RuntimeEnabledFeatures::CompositeAfterPaintEnabled())
+      return false;
     if (!known_to_be_opaque_.has_value())
       known_to_be_opaque_.emplace(CalculateKnownToBeOpaque(record_.get()));
     return *known_to_be_opaque_;