DevTools: Fix heap snapshot grid misalignment after r152691

r152691 has changed behavior of float items in nowrap containers.
Adjust heap snapshot CSS to conform to the new semantics.

BUG=248995
R=apavlov@chromium.org

Review URL: https://codereview.chromium.org/17261019

git-svn-id: svn://svn.chromium.org/blink/trunk@152770 bbb929c8-8fbe-4397-9dbb-9b2b20218538