Currently the VisitObject function iterates the object and then colors
it black. This does not work well with concurrent marking. The function
should instead first try to mark the object black and iterate its body
only if the color transition succeeds.

BUG=chromium:694255

Review-Url: https://codereview.chromium.org/2853323003
Cr-Commit-Position: refs/heads/master@{#45095}