Document the need for !IsAtShadowBoundary() in matched styles cache

Some styles (e.g. webkitUserModify) should not propagate from a
ShadowRoot to the children. Note [1] that it is possible all such
properties are deprecated. But they still have decent [2] usage.
See [3] for what happens if this check is removed.

[1] https://crbug.com/687462
[2] https://chromestatus.com/metrics/css/timeline/popularity/338
[3] https://ci.chromium.org/p/chromium/builders/try/linux-rel/570174?

Fixed: 1158424
Change-Id: I994691cb63d96d60d8a47d8698ad86a43f8ea10e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2593285
Auto-Submit: Mason Freed <masonfreed@chromium.org>
Commit-Queue: Anders Hartvoll Ruud <andruud@chromium.org>
Reviewed-by: Anders Hartvoll Ruud <andruud@chromium.org>
Cr-Commit-Position: refs/heads/master@{#837469}

third_party/blink/renderer/core/css/resolver/style_resolver.cc

@@ -1376,7 +1376,8 @@ StyleResolver::CacheSuccess StyleResolver::ApplyMatchedCache(
     // earlier style object built using the same exact style declarations. We
     // then only need to apply the inherited properties, if any, as their values
     // can depend on the element context. This is fast and saves memory by
-    // reusing the style data structures.
+    // reusing the style data structures. Note that we cannot do this if the
+    // direct parent is a ShadowRoot.
     if (state.ParentStyle()->InheritedDataShared(
             *cached_matched_properties->parent_computed_style) &&
         !IsAtShadowBoundary(&element)) {