title: sizeWithFont in MultiThread Crash!
body: <p>sizeWithFont crashed in multithread,this is the debug info:</p>
  
  <pre><code>1 0x00a0df8e in icu::RuleBasedBreakIterator::handleNext
  2 0x00a0daff in icu::RuleBasedBreakIterator::next
  3 0x00a0d174 in icu::RuleBasedBreakIterator::following
  4 0x35879719 in WebCore::nextBreakablePosition
  5 0x3587842a in -[NSString(WebStringDrawing) _web_drawInRect:withFont:ellipsis:alignment:lineSpacing:includeEmoji:measureOnly:]
  6 0x35877da3 in -[NSString(WebStringDrawing) _web_sizeInRect:withFont:ellipsis:lineSpacing:]
  7 0x3090d238 in -[NSString(UIStringDrawing) sizeWithFont:constrainedToSize:lineBreakMode:lineSpacing:]
  8 0x3090cee3 in -[NSString(UIStringDrawing) sizeWithFont:constrainedToSize:lineBreakMode:]
  </code></pre>
  
  <p>now I solve the error by using a NSLock object,before using this function I will lock this object,and after that unlock it</p>
  
  <p>but I think there must be a more better solution! </p>
  
  <p>and  I found this error only appeared when the NSString object for this function on both two threads are multi-lines text</p>