TextBreakIteratorCache shouldn't be used from worker threads.

https://bugs.webkit.org/show_bug.cgi?id=260498
<rdar://113654067>

Reviewed by David Kilzer.

An alternative here would be to have a cache per-thread, but I'm not aware of an easy way to
store thread local data from WTF currently.

This just skips the cache for non-main threads, and we can consider adding it back as needed
for performance in the future.

* Source/WTF/wtf/text/TextBreakIterator.h:
(WTF::CachedTextBreakIterator::CachedTextBreakIterator):
(WTF::CachedTextBreakIterator::~CachedTextBreakIterator):

Canonical link: https://commits.webkit.org/267175@main

Source/WTF/wtf/text/TextBreakIterator.h

@@ -96,6 +96,7 @@ class TextBreakIterator {
     }
 
 private:
+    friend class CachedTextBreakIterator;
     friend class TextBreakIteratorCache;
 
     using Backing = std::variant<TextBreakIteratorICU, TextBreakIteratorPlatform>;
@@ -151,6 +152,7 @@ class TextBreakIteratorCache {
 
     TextBreakIterator take(StringView string, const UChar* priorContext, unsigned priorContextLength, TextBreakIterator::Mode mode, TextBreakIterator::ContentAnalysis contentAnalysis, const AtomString& locale)
     {
+        ASSERT(isMainThread());
         auto iter = std::find_if(m_unused.begin(), m_unused.end(), [&](TextBreakIterator& candidate) {
             return candidate.mode() == mode && candidate.contentAnalysis() == contentAnalysis && candidate.locale() == locale;
         });
@@ -164,6 +166,7 @@ class TextBreakIteratorCache {
 
     void put(TextBreakIterator&& iterator)
     {
+        ASSERT(isMainThread());
         m_unused.append(WTFMove(iterator));
         if (m_unused.size() > capacity)
             m_unused.remove(0);
@@ -181,13 +184,13 @@ class CachedTextBreakIterator {
     WTF_MAKE_FAST_ALLOCATED;
 public:
     CachedTextBreakIterator(StringView string, const UChar* priorContext, unsigned priorContextLength, TextBreakIterator::Mode mode, const AtomString& locale, TextBreakIterator::ContentAnalysis contentAnalysis = TextBreakIterator::ContentAnalysis::Mechanical)
-        : m_backing(TextBreakIteratorCache::singleton().take(string, priorContext, priorContextLength, mode, contentAnalysis, locale))
+        : m_backing(isMainThread() ? TextBreakIteratorCache::singleton().take(string, priorContext, priorContextLength, mode, contentAnalysis, locale) : TextBreakIterator(string, priorContext, priorContextLength, mode, contentAnalysis, locale))
     {
     }
 
     ~CachedTextBreakIterator()
     {
-        if (m_backing)
+        if (m_backing && isMainThread())
             TextBreakIteratorCache::singleton().put(WTFMove(*m_backing));
     }