Fix comment about runtime profile serialization.
Fix comment about runtime profile serialization.

Fixes the comment about how the runtime profile tree is flattened using
pre-order traversal instead of in-order traversal. The implementation in
RuntimeProfile::ToThrift() shows exactly that.

Change-Id: Ib6c3dc7506a14d6b1d467177669b6d701ffedd45
Reviewed-on: http://gerrit.cloudera.org:8080/615
Tested-by: Internal Jenkins
Reviewed-by: Martin Grund <2546548d516fa64c411fde8242af35f3f80ad31f@cloudera.com>

cloudera/Impala,michaelhkw/incubator-impala,cloudera/Impala,cloudera/Impala,michaelhkw/incubator-impala,cloudera/Impala,michaelhkw/incubator-impala,michaelhkw/incubator-impala,michaelhkw/incubator-impala,michaelhkw/incubator-impala,cloudera/Impala,michaelhkw/incubator-impala,cloudera/Impala,cloudera/Impala