Bug 1321144 - add some operators for WRExternalImageId and WRImageKey. r=gfx?

MozReview-Commit-ID: 441DP6hn2G5

gfx/webrender_bindings/src/webrender.h

@@ -25,6 +25,10 @@ struct WRImageKey {
   bool operator==(const WRImageKey& aRhs) const {
     return a == aRhs.a && b == aRhs.b;
   }
+
+  bool operator<(const WRImageKey& aRhs) const {
+    return (a < aRhs.a) || ((a == aRhs.a) && (b < aRhs.b));
+  }
 };
 
 struct WRRect {
@@ -53,6 +57,14 @@ struct WRImageMask
 typedef uint64_t WRImageIdType;
 struct WRExternalImageId {
   WRImageIdType id;
+
+  bool operator==(const WRExternalImageId& aRhs) const {
+    return id == aRhs.id;
+  }
+
+  bool operator<(const WRExternalImageId& aRhs) const {
+    return id < aRhs.id;
+  }
 };
 
 enum WRExternalImageType {