Try to parse json files on presubmit and add warning it if unable to
parse.

R=apolito@google.com

Bug: 1223923
Change-Id: I67ab4702cb9b995aace6527f4a04f1a1068db7d5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3019692
Auto-Submit: Josip Sokcevic <sokcevic@google.com>
Commit-Queue: Anthony Polito <apolito@google.com>
Reviewed-by: Anthony Polito <apolito@google.com>