PRESUBMIT.py improvements

* Updated code to check for owners in reviewers for Gerrit issues instead for in the TBR= line.
* The previous TBR parsing code was not accounting for adding comments. Eg: "TBR=rmistry (Testing only)".

BUG=skia:5825

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2981

Change-Id: I910a3ae71a9f57c14f80c0b0404041cbe451a77c
Reviewed-on: https://skia-review.googlesource.com/2981
Reviewed-by: Joe Gregorio <jcgregorio@google.com>
Commit-Queue: Ravi Mistry <rmistry@google.com>
1 file changed