Add a PRESUBMIT script for LSan suppressions.

Also make the comments in the suppressions file more noticeable.

BUG=NONE
R=timurrrr@chromium.org
NOTRY=true

Review URL: https://codereview.chromium.org/26490002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@227515 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 227515