This happens for both -opengl desktop and -opengl dynamic builds. There's an ordering problem in how we initialize our OpenGL extensions.
The logic in GpuInit::InitializeInProcess() (see https://code.woboq.org/qt5/qtwebengine/src/3rdparty/chromium/gpu/ipc/service/gpu_init.cc.html ) is the following:
THat is, we first initialize OpenGL without extensions, then try to query GPU features, and then only initialize extensions.
However, ComputeGpuFeatureInfo does create an OpenGL surface for querying the extensions, and on Windows this OpenGL context requires wglCreatePbufferARB to be available (see GLSurfaceQtWGL::Initialize).