Initialize all ImageCandidate's member variables

While re-reading the code, I noticed that some of ImageCandidate's member variables
are not initialized as they should.
This CL fixes that.

BUG=

Review URL: https://codereview.chromium.org/311053011

git-svn-id: svn://svn.chromium.org/blink/trunk@175758 bbb929c8-8fbe-4397-9dbb-9b2b20218538