{AtomicValue}: A simple integer value that can be atomically
read/set/incremented.

{AtomicEnumSet}: Similar to EnumSet this set basically represents an atomic
bitfield based on enums. Atomic operations guarantee that setting one bit does
not race with setting other bits.

{AtomicEnumFlag}: A flag that is based on an enum that can be read and
(attempted to be) changed atomically.

BUG=chromium:524425
LOG=N

Review URL: https://codereview.chromium.org/1310993004

Cr-Commit-Position: refs/heads/master@{#30560}