Experiment: check if cssTextCache() needs ThreadSpecific<>.

third_party/WebKit/Source/core/css/CSSPrimitiveValue.cpp

Index: third_party/WebKit/Source/core/css/CSSPrimitiveValue.cpp
diff --git a/third_party/WebKit/Source/core/css/CSSPrimitiveValue.cpp b/third_party/WebKit/Source/core/css/CSSPrimitiveValue.cpp
index 35c241c8ef912f2ddebc27f7d7a197bc78d7803a..690455f4c085deef47adb85c4c032fa029d678e5 100644
--- a/third_party/WebKit/Source/core/css/CSSPrimitiveValue.cpp
+++ b/third_party/WebKit/Source/core/css/CSSPrimitiveValue.cpp
@@ -160,6 +160,8 @@ bool CSSPrimitiveValue::colorIsDerivedFromElement() const
 using CSSTextCache = WillBePersistentHeapHashMap<RawPtrWillBeWeakMember<const CSSPrimitiveValue>, String>;
 static CSSTextCache& cssTextCache()
 {
+    // Experiment: verify that this cache is used from non-main threads.
+    RELEASE_ASSERT(isMainThread());
     AtomicallyInitializedStaticReference(ThreadSpecific<CSSTextCache>, cache, new ThreadSpecific<CSSTextCache>());
     return *cache;
 }