Bug 1540015 - part 2: Make nsPresContext.h stop including nsIPresShel.h and nsIPresShellInlines.h r=smaug

`*Inlines.h` shouldn't be included by another header file, but `nsPresContext.h`
does it.  This causes include-hell which blocks the following fix.

Additionally, it causes an include hell between `PresShell.h` vs.
`nsIPresShell.h` and `nsPresContext.h if `Document.h` includes `PresShell.h`.
Therefore, this patch also solves this include hell with adding
`nsPresContextInlines.h`.

Differential Revision: https://phabricator.services.mozilla.com/D25333

--HG--
extra : moz-landing-system : lando