Compute & report timing information for line searches.

- We now compute & report the cumulative time spent performing the
  following tasks as part of a line search:
  - Evaluation of the univariate cost function value & gradient.
  - Minimization of the interpolating polynomial.
  - Total time spent performing line searches.
- This information is now reported for all minimizers, although only in
  the case of a constrained problem for the TR minimizer.
- Remove LineSearch::Function abstraction in place of using
  LineSearchFunction implementation directly, and remove virtual
  functions from LineSearchFunction.
-- LineSearch::Function added an unnecessary level of abstraction since
   the user always had to create a LineSearchFunction anyway to use a
   Ceres Evaluator, and it added an unncessary virtual function call.

Change-Id: Ia4e1921d78f351ae119875aa97a3ea5e8b5d9877

include/ceres/gradient_problem_solver.h

@@ -303,6 +303,10 @@ class CERES_EXPORT GradientProblemSolver {
     // Time (in seconds) spent evaluating the gradient.
     double gradient_evaluation_time_in_seconds;
 
+    // Time (in seconds) spent minimizing the interpolating polynomial
+    // to compute the next candidate step size as part of a line search.
+    double line_search_polynomial_minimization_time_in_seconds;
+
     // Number of parameters in the probem.
     int num_parameters;