Revert "Remove --readPath/-r from DM"

This reverts commit c5245fcd8ec275e76f5302d629034a344a4da581.

Reason for revert: Still being used.

Original change's description:
> Remove --readPath/-r from DM
> 
> This was an (unused) feature to compare GM results against a previously
> generated set, while DM was running. It was also one of the users of
> JsonCPP, which is being evicted from Skia.
> 
> Bug: skia:
> Change-Id: I42d6204250782681e22219863e7170744d8be111
> Reviewed-on: https://skia-review.googlesource.com/c/186867
> Auto-Submit: Brian Osman <brianosman@google.com>
> Commit-Queue: Mike Klein <mtklein@google.com>
> Reviewed-by: Mike Klein <mtklein@google.com>

TBR=mtklein@google.com,brianosman@google.com

Change-Id: I05a74166a72ce106be73f3e0809e7f96e963aab5
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: skia:
Reviewed-on: https://skia-review.googlesource.com/c/186868
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
3 files changed