Bug 1545342 - part 2: Make some public enum of nsIPresShell move to mozilla namespace and defined as enum class in PresShellForwards.h r=smaug

This patch moves some `enum` in `nsIPresShell` which are in public scope into
`mozilla` namespace and change them as `enum class`es.

Unfortunately, only "where to scroll" enum is just defines constants of
percentages of scroll destination.  Therefore, this patch makes only them
as `static const`.

Differential Revision: https://phabricator.services.mozilla.com/D28606

dom/html/nsGenericHTMLFrameElement.cpp

@@ -296,7 +296,7 @@ nsresult nsGenericHTMLFrameElement::AfterSetAttr(
                 presShell ? presShell->GetRootScrollFrame() : nullptr;
             if (rootScroll) {
               presShell->FrameNeedsReflow(
-                  rootScroll, nsIPresShell::eStyleChange, NS_FRAME_IS_DIRTY);
+                  rootScroll, IntrinsicDirty::StyleChange, NS_FRAME_IS_DIRTY);
             }
           }
         }