Avoid unnecessary updateActiveStyle comparing shadow styles.

third_party/WebKit/Source/core/css/resolver/ScopedStyleResolver.h

Index: third_party/WebKit/Source/core/css/resolver/ScopedStyleResolver.h
diff --git a/third_party/WebKit/Source/core/css/resolver/ScopedStyleResolver.h b/third_party/WebKit/Source/core/css/resolver/ScopedStyleResolver.h
index 2ca625c22f1b4fa36a6ab4a1ef368714d70e3caa..dad35f41afa55895ae63ac59d0ffec7533ecb5f2 100644
--- a/third_party/WebKit/Source/core/css/resolver/ScopedStyleResolver.h
+++ b/third_party/WebKit/Source/core/css/resolver/ScopedStyleResolver.h
@@ -76,6 +76,8 @@ class ScopedStyleResolver final : public GarbageCollected<ScopedStyleResolver> {
   void setNeedsAppendAllSheets() { m_needsAppendAllSheets = true; }
   static void keyframesRulesAdded(const TreeScope&);
   static ContainerNode& invalidationRootForTreeScope(const TreeScope&);
+  CORE_EXPORT static bool hasSameStyles(const ScopedStyleResolver*,

[sof, 2017/01/03 18:48:34]

s/has/have/, but why not an equality method rather than a static binary
predicate?

[rune, 2017/01/03 18:52:31]

You mean non-static ScopedStyleResolver::hasSameStyles(const
ScopedStyleResolver*) ?

[sof, 2017/01/03 19:11:17]

ok (wrt static.) Basic question -- are these author stylesheets ordered somehow
that this basic pointer equality checking is sufficiently precise to determine
"same"ness?

[rune, 2017/01/03 20:51:32]

The stylesheets are in DOM tree order for the given TreeScope. Quite often web
components only contain a single style element whose StyleSheetContents will be
shared between multiple instances of the same component because of the String
<-> StyleSheetContents cache in StyleEngine. Even if there are more than one
style element, the ordering will be the same in multiple instances of the same
component. The sameness is used to decide if we can share ComputedStyle objects
between different shadow hosts.

[rune, 2017/01/03 21:33:01]

Changed has -> have.

+                                        const ScopedStyleResolver*);
 
   DECLARE_TRACE();