This patch creates new header, `mozilla/PresShellForwards.h`. It should have
all forward declarations of global class/struct in `nsIPresShell.h` and
`mozilla/PresShell.h`.
Additionally, this moves all `enum`s and `constant`s in them into the new file
with changing them to `enum class`es.
This will make other headers which require only specific types in the header
files not include them.
Differential Revision: https://phabricator.services.mozilla.com/D28605
--HG--
extra : moz-landing-system : lando
Masayuki Nakano