Extend BM_InEdgeIteration to also cover the case where EdgeSet spills into std::set.

Add corresponding benchmarks for ConvertGraphDefToGraph.

Run on xxx (12 X 3494 MHz CPUs); 2018-10-25T16:37:34.542204137-07:00
CPU: Intel Haswell with HyperThreading (6 cores) dL1:32KB dL2:256KB dL3:15MB
Benchmark                   Time(ns)        CPU(ns)     Iterations
------------------------------------------------------------------
BM_InEdgeIteration/10/2          186            186        3744358
BM_InEdgeIteration/64/2          597            598        1000000
BM_InEdgeIteration/512/2        4752           4758         145728
BM_InEdgeIteration/4k/2        56360          56428          10000
BM_InEdgeIteration/32k/2      420538         421091           1628
BM_InEdgeIteration/10/4          315            315        2225163
BM_InEdgeIteration/64/4          751            752         933728
BM_InEdgeIteration/512/4        6104           6109         100000
BM_InEdgeIteration/4k/4        58948          59007          10000
BM_InEdgeIteration/32k/4      562371         563249           1000
BM_InEdgeIteration/10/8          722            723         966333
BM_InEdgeIteration/64/8         2784           2791         253673
BM_InEdgeIteration/512/8       55480          55562          10000
BM_InEdgeIteration/4k/8       251679         252052           2763
BM_InEdgeIteration/32k/8     3838108        3844486            196
BM_InEdgeIteration/10/16        1553           1555         443749
BM_InEdgeIteration/64/16        6327           6335         100000
BM_InEdgeIteration/512/16     100447         100573           6936
BM_InEdgeIteration/4k/16      541743         542511           1000
BM_InEdgeIteration/32k/16    7918551        7930790             89
BM_GraphCreation/10/2          70604          91562           7576
BM_GraphCreation/64/2         161531         184322           3792
BM_GraphCreation/512/2       1625739        1665700            451
BM_GraphCreation/4k/2       65684409       65786553             10
BM_GraphCreation/32k/2    3207273777     3210972335              1
BM_GraphCreation/10/4          93866         114788           5878
BM_GraphCreation/64/4         211641         232140           3070
BM_GraphCreation/512/4       1989221        2028664            365
BM_GraphCreation/4k/4       62165818       62302848             10
BM_GraphCreation/32k/4    3197463931     3201873889              1
BM_GraphCreation/10/8         145534         167901           4194
BM_GraphCreation/64/8         351864         379019           1866
BM_GraphCreation/512/8       3155522        3203574            212
BM_GraphCreation/4k/8       64976006       65111337             10
BM_GraphCreation/32k/8    3406901369     3410783962              1
BM_GraphCreation/10/16        248981         273488           2564
BM_GraphCreation/64/16        601753         634483           1000
BM_GraphCreation/512/16      4757989        4802416            146
BM_GraphCreation/4k/16      84289699       84470976              8
BM_GraphCreation/32k/16   3690183415     3694977106              1
PiperOrigin-RevId: 218778510