This flag was set in QWebEngineView::contextMenuEvent and checked in
QWebEnginePage::contextMenuRequested. The latter would bail out if the
flag was not set.

When the user pressed Shift-F10, the key event was received by
RenderWidgetHostViewQtDelegateWidget::event and forwarded to Chromium.
Blink's default event handler invoked the context menu, and our callback
QWebEnginePage::contextMenuRequested was called. But nothing happened,
because m_pendingContextMenuEvent has never been set.

There is no reason to have this flag. React on every context menu
request just like in the QtQuick implementation.

Now pressing Shift-F10 invokes the context menu on web pages.

Task-number: QTBUG-58306
Change-Id: I2db1b17604e5521e44613297120a964cc4e4d544
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@qt.io>