Now non-atomic color transition operations return a boolean indicating
whether the transition succeeded or not.

This allows to replace color check and transition operations with a
single transition operation. For example:

if (IsWhite(object)) {
  WhiteToBlack(object);
  Foo();
}

becomes

if (WhiteToBlack(object)) {
  Foo();
}

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

Review-Url: https://codereview.chromium.org/2863703004
Cr-Commit-Position: refs/heads/master@{#45136}