The previous implementation of CheckLongLines did not
handle global pylint disable/enable directives properly,
i.e. the difference between:

   # pylint: disable=line-too-long
   .... checks disabled for all lines here.
   # pylint: enable=line-too-long

versus:

   # Check only disabled for the line below
   some python statements   # pylint: disable=line-too-long

This CL changes the implementation to support Python files
properly. Note that in order to not disturb the mock-based
unit-tests, a new function is introduced to be able to
filter the list of affected files based on their file
extension.

BUG=890734
R=mattcary@chromium.org,ehmaldonado@chromium.org,dpranke@chromium.org

Change-Id: Id52deff53913b8d47a4157f42b1fffbd3b103201
Reviewed-on: https://chromium-review.googlesource.com/c/1396094Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>
Commit-Queue: David Turner <digit@chromium.org>