Add random string to heap profile directory.
Add random string to heap profile directory.

Before, we would hit PERFETTO_CHECK(base::Mkdir(temp_dir)) when using in
a loop.

Change-Id: Idcff1537eb6c50ef21a163bb7f8434c4e68e9cc5
google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto