Add benchmark for Graph -> GraphDef conversion.

Benchmark output:
Run on ** redacted ** (72 X 2993 MHz CPUs); 2018-12-13T11:50:10.199225713-08:00
CPU: Intel Skylake Xeon with HyperThreading (36 cores) dL1:32KB dL2:1024KB dL3:24MB
Benchmark              Time(ns)        CPU(ns)     Iterations
-------------------------------------------------------------
BM_ToGraphDef/10/2         5880           5888         100000
BM_ToGraphDef/64/2        18670          18699          37373
BM_ToGraphDef/512/2      125906         126110           5606
BM_ToGraphDef/4k/2      1331662        1333420            538
BM_ToGraphDef/32k/2    13059699       13086186             51
BM_ToGraphDef/10/4         9929           9947          71281
BM_ToGraphDef/64/4        27190          27235          25837
BM_ToGraphDef/512/4      164695         164936           4245
BM_ToGraphDef/4k/4      1794584        1797493            395
BM_ToGraphDef/32k/4    16604391       16641067             41
BM_ToGraphDef/10/8        18191          18212          38647
BM_ToGraphDef/64/8        45134          45191          15412
BM_ToGraphDef/512/8      327305         327803           2177
BM_ToGraphDef/4k/8      2868544        2872056            250
BM_ToGraphDef/32k/8    25948189       25999957             27
BM_ToGraphDef/10/16       34698          34763          20267
BM_ToGraphDef/64/16       78154          78332           8873
BM_ToGraphDef/512/16     595667         596683           1000
BM_ToGraphDef/4k/16     4867696        4877087            142
BM_ToGraphDef/32k/16   46445400       46518659             15
PiperOrigin-RevId: 225410602