QtWebEngine builds got flaky recently in yocto when updating Qt modules from early 5.12 snapshot to proposed beta1 content fail build often with missing mojom header:
There seems to be dependency problem or race condition as the mojom headers sometimes get built and building goes through fine. This exact issue was already present in earlier meta-qt5 update attempt where updating webengine to a revision matched by Qt 5.12.0 snapshot build. Updating webengine further to then-latest revision in 5.12 branch solved the build issue momentarily and a Qt 5.12 snapshot update was succesful in meta-qt5 ( https://codereview.qt-project.org/#/c/241374/7 ). Now, trying to build the latest 5.12.0 beta1 content fails seemingly every time.