[wasm] qt-wasm app's memory keep growing when qt event system is running

    I build qt5-wasm from the latest git 5.15 branch with emscripten 1.39.8 and with pthread enabled. I found that when qt event system is running, qt app's memory consumption keep growing.

    I run a very simple qt-wasm app which simply post event every 40 ms, run it from chrome 84 (ubuntu 20.04), after five minutes this app's memory grow about 82MB (I observe memory consumption from chrome's task manager). Run this app with chrome under Windows I can see similar memory growing.

    I have attached a sample program to reproduce this issue. Start the app in chrome, press start button, you will see memory keep growing with chrome's task manager. Wait for 5 minutes, the memory will grow significantly.

    BTW: Run the same sample program under Ubuntu/Windows (I mean the desktop version), the memory consumption will be a constant value which is expected.