Fix build for Boot2Qt

A change in Chromium 69 pulled a mojom header into
the public content browser API without, assuming the
dependencies would always be fulfilled via building
the extension system.

Add the dependency explicitly to fix this.

Change-Id: I005aa0970c7ecdbdce90d8c1db78c38f2274d324
Fixes: QTBUG-70914
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@qt.io>