Store bugs separately from modifiers.

This simplifies all the code and better matches the syntax of
TestExpectations.

wip

R=dpranke@chromium.org

Review URL: https://codereview.chromium.org/20588002

git-svn-id: svn://svn.chromium.org/blink/trunk@155027 bbb929c8-8fbe-4397-9dbb-9b2b20218538