Implement the DisplayItem id for paint slimming

DisplayItems all have a renderer and (currently) have either a paint
phase or a clip type. This patch adds this storage to DisplayItem.

Major changes in this patch:
1) DisplayItem::Id was introduced which holds a renderer and the display
item type.
2) DisplayItem::Type enum has been introduced which is the union of all
DisplayItem types.
3) Some comments have been added to ensure Type and PaintPhase stay
in sync.

Discussion on the design can be found in:
https://docs.google.com/a/chromium.org/document/d/1iGJdE1hD15ISQKDh-I7m1R3MYiamvTlmsyplxts5n0g/edit#

No new tests as there is no change in behavior. A followup will actually
use this data.

R=chrishtr@chromium.org

Review URL: https://codereview.chromium.org/685343002

git-svn-id: svn://svn.chromium.org/blink/trunk@184662 bbb929c8-8fbe-4397-9dbb-9b2b20218538