Fix uninitialized reads in BreakingContext

There were a couple uninitialized reads in BreakingContext's constructor
that were masked because the values were overwritten in a later
initializeForCurrentObject call. This patch removes the uninitialized reads.

BUG=301500

Review URL: https://codereview.chromium.org/25339002

git-svn-id: svn://svn.chromium.org/blink/trunk@158590 bbb929c8-8fbe-4397-9dbb-9b2b20218538