Rename Document::GetShell() to Document::GetPresShell() and make it return PresShell* rather than nsIPresShell*

This makes `Document::GetShell()` return `PresShell*` instead of `nsIPresShell`.

Additonally, "shell" is unclear ("docshell" vs. "presshell").  Therefore, this
also renames `Document::GetShell()` to `Document::GetPresShell()`.

Similarly, some other method names of `Document` are also renamed from
`*Shell*` to `*PresShell*`.

Differential Revision: https://phabricator.services.mozilla.com/D25338

components/style/gecko/media_queries.rs

@@ -169,6 +169,7 @@ impl Device {
             self.document()
                 .mPresShell
                 .as_ref()?
+                ._base
                 .mPresContext
                 .mRawPtr
                 .as_ref()