Update display list canvas configuration logic

This patch adds a new "force" mode that will always enable
display lists (for testing), and the behavior of "enabled" is
changed to use some decision criteria to decide whether
display lists should be used. This prepares the ground for
making display-list-2d-canvas enabled by default.

BUG=386601

Review URL: https://codereview.chromium.org/693873005

git-svn-id: svn://svn.chromium.org/blink/trunk@184748 bbb929c8-8fbe-4397-9dbb-9b2b20218538