pprof_proto_generator: Set units for common events

This will help people read the graph, particularly with the nanoseconds
view, which confused people often when I showed them.

BUG=201361738
Test: Ran test/test.py -p TestPprofProtoGenerator.\*

Change-Id: I67448febbf5427de31155575826cee3029251a08
2 files changed