The layout tests compositing/lots-of-img-layers.html and compositing/lots-of-img-layers-with-opacity.html are too slow with our MESA layout test backend.
This was related to a Fixed WebKit bug https://bugs.webkit.org/show_bug.cgi?id=49629 but our bug migrator doesn't work any more.
There is some effort to move from MESA to SwiftShader which might change the situation.
Comment 1 by bugdroid1@chromium.org
, Sep 15 2016