Bug 1357754 - Add ScrollDirection IPC serialization code. r=botond

In the next patch we'll need to send ScrollDirection over IPDL so we
need to add serialization code for it.

MozReview-Commit-ID: HK8ibuhhnkS

gfx/ipc/GfxMessageUtils.h

@@ -295,6 +295,14 @@ struct ParamTraits<mozilla::layers::DiagnosticTypes>
              mozilla::layers::DiagnosticTypes::ALL_BITS>
 {};
 
+template <>
+struct ParamTraits<mozilla::layers::ScrollDirection>
+  : public ContiguousEnumSerializer<
+            mozilla::layers::ScrollDirection,
+            mozilla::layers::ScrollDirection::NONE,
+            mozilla::layers::ScrollDirection::SENTINEL>
+{};
+
 /*
 template <>
 struct ParamTraits<mozilla::PixelFormat>

gfx/layers/LayersTypes.h

@@ -302,7 +302,8 @@ private:
 enum class ScrollDirection : uint32_t {
   NONE,
   VERTICAL,
-  HORIZONTAL
+  HORIZONTAL,
+  SENTINEL /* for IPC serialization */
 };
 
 } // namespace layers

gfx/layers/apz/test/gtest/APZTestCommon.h

@@ -268,6 +268,9 @@ public:
     case ScrollDirection::VERTICAL:
       EXPECT_EQ(PANNING_LOCKED_Y, mState);
       break;
+    case ScrollDirection::SENTINEL:
+      MOZ_ASSERT(false, "Invalid value");
+      break;
     }
   }