Adds a script for converting gc nvp output into csv files. Factors out common
code in gc-nvp-trace-processor.py to gc_nvp_common.py to be shared by both
scripts. Fixes a couple of issues in nvp parsing code.

Review URL: https://codereview.chromium.org/1175113008

Cr-Commit-Position: refs/heads/master@{#29141}