concurrent marking is enabled.

This patch adds kAtomicity flag to IncrementalMarking that is set
depending on the concurrent marking compile time flag.

BUG=chromium:694255
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_chromium_rel_ng

Review-Url: https://codereview.chromium.org/2858343003
Cr-Commit-Position: refs/heads/master@{#45138}