Extend tfprof to associate op stats with Python codes.

It's backward compatible. Stats of a source code line
are aggregated from all ops created by that line.

A example.
_TFProfRoot (0us/22.44ms)
  model_analyzer_test.py:149:run_filename_as_m...:none (0us/22.44ms)
    model_analyzer_test.py:33:_run_code_in_main:none (0us/22.44ms)
      model_analyzer_test.py:208:<module>:test.main() (0us/22.44ms)
        model_analyzer_test.py:132:testComplexCodeView:x = lib.BuildFull... (0us/22.44ms)
          model_analyzer_testlib.py:63:BuildFullModel:return sgd_op.min... (0us/21.83ms)
          model_analyzer_testlib.py:54:BuildFullModel:seq.append(array_... (0us/254us)
            model_analyzer_testlib.py:42:BuildSmallModel:x = nn_ops.conv2d... (0us/134us)
            ...
          model_analyzer_testlib.py:61:BuildFullModel:loss = nn_ops.l2_... (0us/28us)
        model_analyzer_test.py:134:testComplexCodeView:sess.run(variable... (0us/0us)
Change: 155393864