Slight tweaks to the test expectation syntax and warnings

This patch makes two changes to how we parse the TestExpectations
syntax and generate warnings.

First, it is no longer an error to have both "Slow" and "Timeout".
There's perfectly reasonable case where you might want to mark both
and this shouldn't be an error.

Second, whether lines need to have a "Bug(x)" field or not used to
be a port-specific setting in WebKit, but we no longer need this in
Blink. Moreover, it's annoying to have to make up fake Bug identifiers
if you are using local expectations files, so this patch changes things
so that we only complain about missing bug id's if you are linting the
file (and not when just parsing it or running tests).

R=ojan@chromium.org
BUG=none

Review URL: https://chromiumcodereview.appspot.com/22266006

git-svn-id: svn://svn.chromium.org/blink/trunk@155571 bbb929c8-8fbe-4397-9dbb-9b2b20218538