Bug 1352763 part 1 - Constify RawServoStyleSheet. r=heycam

MozReview-Commit-ID: IXWGobgV59f

--HG--
extra : rebase_source : b569e1e7d30f00f300d91e465048c54024ab82f2

layout/style/ServoStyleSheet.h

@@ -32,7 +32,7 @@ struct ServoStyleSheetInner : public StyleSheetInfo
                        ReferrerPolicy aReferrerPolicy,
                        const dom::SRIMetadata& aIntegrity);
 
-  RefPtr<RawServoStyleSheet> mSheet;
+  RefPtr<const RawServoStyleSheet> mSheet;
 };
 
 
@@ -66,10 +66,10 @@ public:
    */
   void LoadFailed();
 
-  RawServoStyleSheet* RawSheet() const {
+  const RawServoStyleSheet* RawSheet() const {
     return Inner()->mSheet;
   }
-  void SetSheetForImport(RawServoStyleSheet* aSheet) {
+  void SetSheetForImport(const RawServoStyleSheet* aSheet) {
     MOZ_ASSERT(!Inner()->mSheet);
     Inner()->mSheet = aSheet;
   }