REG: QtWebEngine / Pdfwidgets no longer supports plugins

Steps to reproduce:

1. configure Qt with qtdoc, make sure plugins are enabled in qtbase
2. checkout qtwebengine 2af8c8c0b35bd37ba8df591d928720d2b894e816
3. cd ../build/qtwebengine && rm -rf *
4. enable the following features (CLI or Qt Creator):
   -DBUILD_qtwebengine:BOOL=ON
   -DBUILD_qtwebengine:BOOL=ON
   -DFEATURE_qtwebengine_build:BOOL=ON
   -DFEATURE_qtwebengine_build:BOOL=ON
   -DFEATURE_qtwebengine_core_build:BOOL=ON
   -DFEATURE_qtwebengine_core_build:BOOL=ON
   -DFEATURE_qtwebengine_quick_build:BOOL=ON
   -DFEATURE_qtwebengine_quick_build:BOOL=ON
   -DFEATURE_qtwebengine_widgets_build:BOOL=ON
   -DFEATURE_qtwebengine_widgets_build:BOOL=ON
   -DFEATURE_qtpdf_build:BOOL=ON
   -DFEATURE_qtpdf_build:BOOL=ON
   -DFEATURE_qtpdf_quick_build:BOOL=ON
   -DFEATURE_qtpdf_quick_build:BOOL=ON
   -DFEATURE_qtpdf_widgets_build:BOOL=ON
   -DFEATURE_qtpdf_widgets_build:BOOL=ON
   -DFEATURE_webengine_printing_and_pdf:BOOL=ON
   -DFEATURE_webengine_printing_and_pdf:BOOL=ON
5. ninja documentviewer && ninja pdfviewer

Expected result

pdfviewer builds.

Actual result

CMake Error at /home/axel/QtGit/toplevel/dev/qtwebengine/cmake/Gn.cmake:80 (message):
  

  -- GN FAILED

  ERROR at //pdf/BUILD.gn:235:5: Assertion failed.

      assert(enable_plugins, "The PDF viewer relies on plugin infrastructure")
      ^-----

  The PDF viewer relies on plugin infrastructure

  See //chrome/BUILD.gn:1474:5: which caused the file to be included.

      "//pdf",
      ^------

  



  1



ninja: build stopped: subcommand failed.

1. ll ../build/qtdoc/examples/demos/documentviewer/app
   The output...

   libabstractviewer.so
   libjsonviewer.so
   libtxtviewer.so
   

   ...proves that plugins are enabled, so txtviewer and jsonviewer were built, while pdfviewer fails.