Bug 1322570 Part 1 - Add MOZ_ASSERT in nsStyleContext::GetParent() to disallow usage by stylo. r=bz

Also, add GetParentAllowServo() to whitelist the usage in servo-backed style
system in subsequent parts so that we could revisit them later.

MozReview-Commit-ID: JthU18Ftx02

--HG--
extra : rebase_source : b8840adcc4959f96cd429db65584f3e83c09c597

layout/style/nsStyleContext.h

@@ -163,7 +163,15 @@ public:
 #endif
   }
 
-  nsStyleContext* GetParent() const { return mParent; }
+  nsStyleContext* GetParent() const {
+    MOZ_ASSERT(mSource.IsGeckoRuleNode(),
+               "This should be used only in Gecko-backed style system!");
+    return mParent;
+  }
+
+  nsStyleContext* GetParentAllowServo() const {
+    return mParent;
+  }
 
   nsIAtom* GetPseudo() const { return mPseudoTag; }
   mozilla::CSSPseudoElementType GetPseudoType() const {