Correct conversion from flowthread to visual coordinate space when there's border/padding.

If an inner multicol container has border or padding, just using the outer
block offset stored in the inner flow thread won't be good enough. We need to
find the actual first fragmentainer group in the first column set.

Or, put simply: let's do what the comment in the code actually says!

Review URL: https://codereview.chromium.org/1847343003

Cr-Commit-Position: refs/heads/master@{#384737}
3 files changed