2018-08-21  Carlos Garcia Campos  <cgarcia@igalia.com>

        Do not try to update the compositing policy when not in accelerated compositing mode

        https://bugs.webkit.org/show_bug.cgi?id=188787

        Reviewed by NOBODY (OOPS!).

        RenderLayerCompositor::updateCompositingPolicy() is called very often (called from

        RenderLayerCompositor::cacheAcceleratedCompositingFlags()) and it uses WTF::memoryFootprint() to decide the

        current compositing policy. Getting the memory footprint is an expensive operation in Linux (and I suspect other

        non-cocoa ports too), causing an excessive CPU usage. This caused the WPE and GTK+ unit test

        /webkit/WebKitWebContext/uri-scheme to start timing out in the bots, because the test expects things to happen

        fast and that's no longer the case. We could reduce the CPU usage a lot by not trying to update the policy when

        not in accelerated compositing mode. We will need a solution for the accelerated compositing mode, though.

        Fixes WPE/GTK+ unit test /webkit/WebKitWebContext/uri-scheme.

        * rendering/RenderLayerCompositor.cpp:

        (WebCore::RenderLayerCompositor::updateCompositingPolicy): Return early when not in accelerated compositing mode.

    if (!inCompositingMode())

        return false;