title: `usedJSHeapSize` does not track with application memory usage
body: <p>Using <code>window.performance.memory.usedJSHeapSize</code>, I am attempting to monitor the memory consumption of my web application in real time.</p>
  
  <p>There is a large discrepancy between the value returned by the performance API and the value the Chrome Timeline shows that I do not understand.</p>
  
  <p>At application start both are about <code>20mb</code>.</p>
  
  <p>After some navigating around meant to detect a memory leak, Chrome Timeline shows much more usage (say, 150mb), and <code>window.performance.memory.usedJSHeapSize</code> changes very slightly (say,  20.69mb).</p>
  
  <p>Is this value measuring something different than I think it's measuring? Why the discrepancy?</p>