Issue #1629 - Part 2: Implement the Explicitly Enabled flag.

This part of the bug was significantly complicated by the following major refactors:

https://bugzilla.mozilla.org/show_bug.cgi?id=1456435
https://bugzilla.mozilla.org/show_bug.cgi?id=1459498

As best as I can tell, we just need to implement the explicitly enabled
flag on every instance of GetStyleSheetInfo, make sure
aIsExplicitlyEnabled is false in every situation except the one where
the disabled content attribute is removed from a link element, and
enable alternate stylesheets if this flag is set on them. So we take the
explicitly enabled flag as an input to PrepareSheet, and also add it to
LoadStyleLink and LoadInlineStyle. I also decided not to defer loading of
alternate stylesheets that have been explicitly enabled.

dom/html/HTMLStyleElement.cpp

@@ -223,12 +223,14 @@ HTMLStyleElement::GetStyleSheetInfo(nsAString& aTitle,
                                     nsAString& aType,
                                     nsAString& aMedia,
                                     bool* aIsScoped,
-                                    bool* aIsAlternate)
+                                    bool* aIsAlternate,
+                                    bool* aIsExplicitlyEnabled)
 {
   aTitle.Truncate();
   aType.Truncate();
   aMedia.Truncate();
   *aIsAlternate = false;
+  *aIsExplicitlyEnabled = false;
 
   nsAutoString title;
   GetAttr(kNameSpaceID_None, nsGkAtoms::title, title);