Rename DisplayItemList to PaintController

Rename DisplayItemList to PaintController

This patch renames DisplayItemList to PaintController because the paint
controller actually contains two lists of display items and handles
updating them each frame (the merge/commit step). The PaintController
owns two DisplayItems objects for the actual lists.

This patch is a followup to [1] which factored DisplayItems (the list)
out of DisplayItemList. A followup patch will rename DisplayItems to
DisplayItemList to make this clear.

[1] https://crrev.com/5243078103e8ab4753cfbf3b46058598c76931f0

BUG=537409
Committed: https://crrev.com/f92b760fedf42ff985d34e57e048e0b013ab0902
Cr-Commit-Position: refs/heads/master@{#353947}

[pdr., 2015-10-13 23:32:54]

pdr@chromium.org changed reviewers:
+ chrishtr@chromium.org, jbroman@chromium.org

[chrishtr, 2015-10-13 23:36:01]

The CQ bit was checked by chrishtr@chromium.org

[chrishtr, 2015-10-13 23:36:02]

lgtm

[commit-bot: I haz the power, 2015-10-13 23:38:21]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1401363003/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1401363003/1

[commit-bot: I haz the power, 2015-10-14 02:26:37]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-10-14 02:27:21]

Patchset 1 (id:??) landed as
https://crrev.com/f92b760fedf42ff985d34e57e048e0b013ab0902
Cr-Commit-Position: refs/heads/master@{#353947}