Default Initialize ExpressionRef to Zero

The default constructor of ExpressionRef now creates a compile
time constant zero assignment. This patch is required, because
a reason change in Jet assumes default 0 initialization.

Change-Id: I8185cef587c17ab828896bce1e768170f8229d4e

include/ceres/codegen/internal/expression_ref.h

@@ -46,7 +46,7 @@ namespace internal {
 //
 // ExpressionRef should be passed by value.
 struct ExpressionRef {
-  ExpressionRef() = default;
+  ExpressionRef() : ExpressionRef(0.0) {}
 
   // Create a compile time constant expression directly from a double value.
   // This is important so that we can write T(3.14) in our code and
@@ -57,6 +57,10 @@ struct ExpressionRef {
   // must work for T = Jet<ExpressionRef>.
   ExpressionRef(double compile_time_constant);
 
+  // Adds the expression to the active graph and initializes this ExpressionRef
+  // accordingly.
+  ExpressionRef(const Expression& expression);
+
   // By adding this deleted constructor we can detect invalid usage of
   // ExpressionRef. ExpressionRef must only be created from constexpr doubles.
   //
@@ -97,8 +101,6 @@ struct ExpressionRef {
 
   // The index into the ExpressionGraph data array.
   ExpressionId id = kInvalidExpressionId;
-
-  static ExpressionRef Create(ExpressionId id);
 };
 
 // A helper function which calls 'InsertBack' on the currently active graph.