Define symbols properly

The problem popped up when passing the constants by reference
(https://chromium-review.googlesource.com/c/565141).
It's a bit ugly, but, the C++11 standard requires a definition
additionally to the existing declaration in the body of the class:

9.4.2/4: If a static data member is of const literal type, its
  declaration in the class definition can specify a
  brace-or-equal-initializer in which every initializer-clause that is
  an assignment-expression is a constant expression. A static data
  member of literal type can be declared in the class definition with
  the constexpr specifier; if so, its declaration shall specify a
  brace-or-equal-initializer in which every initializer-clause that i
  an assignment-expression is a constant expression. [Note: In both
  these cases, the member may appear in constant expressions. — end
  note] The member shall still be defined in a namespace scope if it is
  odr-used (3.2) in the program and the namespace scope definition shall
  not contain an initializer.

Drive-by: Make the static constants constexpr.

R=bmeurer@chromium.org

Change-Id: Idc3d20bf2adf31d874c23ff8bfec52437789160a
Reviewed-on: https://chromium-review.googlesource.com/567506Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46599}

src/elements.cc

@@ -95,12 +95,14 @@ template<ElementsKind Kind> class ElementsKindTraits {
   typedef FixedArrayBase BackingStore;
 };
 
-#define ELEMENTS_TRAITS(Class, KindParam, Store)               \
-template<> class ElementsKindTraits<KindParam> {               \
- public:   /* NOLINT */                                        \
-  static const ElementsKind Kind = KindParam;                  \
-  typedef Store BackingStore;                                  \
-};
+#define ELEMENTS_TRAITS(Class, KindParam, Store)    \
+  template <>                                       \
+  class ElementsKindTraits<KindParam> {             \
+   public: /* NOLINT */                             \
+    static constexpr ElementsKind Kind = KindParam; \
+    typedef Store BackingStore;                     \
+  };                                                \
+  constexpr ElementsKind ElementsKindTraits<KindParam>::Kind;
 ELEMENTS_LIST(ELEMENTS_TRAITS)
 #undef ELEMENTS_TRAITS
 

src/profiler/heap-snapshot-generator.cc

@@ -172,14 +172,18 @@ namespace {  // Avoid littering the global namespace.
 template <size_t ptr_size> struct SnapshotSizeConstants;
 
 template <> struct SnapshotSizeConstants<4> {
-  static const int kExpectedHeapGraphEdgeSize = 12;
-  static const int kExpectedHeapEntrySize = 28;
+  static constexpr int kExpectedHeapGraphEdgeSize = 12;
+  static constexpr int kExpectedHeapEntrySize = 28;
 };
+constexpr int SnapshotSizeConstants<4>::kExpectedHeapGraphEdgeSize;
+constexpr int SnapshotSizeConstants<4>::kExpectedHeapEntrySize;
 
 template <> struct SnapshotSizeConstants<8> {
-  static const int kExpectedHeapGraphEdgeSize = 24;
-  static const int kExpectedHeapEntrySize = 40;
+  static constexpr int kExpectedHeapGraphEdgeSize = 24;
+  static constexpr int kExpectedHeapEntrySize = 40;
 };
+constexpr int SnapshotSizeConstants<8>::kExpectedHeapGraphEdgeSize;
+constexpr int SnapshotSizeConstants<8>::kExpectedHeapEntrySize;
 
 }  // namespace