Make LineSearchMinizer work correctly with negative valued functions.

When reasoning about the function_tolerance based convergence,
LineSearchMinimizer assumed that the objective function is
positive. This used to be the case when LineSearchMinimizer was used
for minimizing non-linear least squares problems. However, with
GradientProblemSolver, the objective function can be negative (for
example when maximizing a function).

This change the minimizer to use the absolute value of the change
from one iteration to another.

https://github.com/ceres-solver/ceres-solver/issues/478

Change-Id: I831e2db96b092374e167c582ab1480b1831d5650