Remove blink::RuntimeEnabledFeatures::slimmingPaintDisplayItemCache

Remove blink::RuntimeEnabledFeatures::slimmingPaintDisplayItemCache

The flag was added to allow us test the mode with display item cache
enabled or disabled. It seems that we haven't been using it for some
time, so perhaps it's time to remove it and always enable the cache.

This is the chromium side change paired with the blink side change
https://codereview.chromium.org/1018053003/.

This is a revert of content/renderer/gpu/render_widget_compositor.cc
in https://codereview.chromium.org/913163004.

TBR=chrishtr@chromium.org

Committed: https://crrev.com/8368b0bf31d5a9fe357c60de91f6c83b08b78267
Cr-Commit-Position: refs/heads/master@{#321440}

[Xianzhu, 2015-03-19 20:26:08]

The CQ bit was checked by wangxianzhu@chromium.org

[commit-bot: I haz the power, 2015-03-19 20:26:45]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1012383005/20001

[commit-bot: I haz the power, 2015-03-19 21:54:57]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-03-19 21:55:50]

Patchset 2 (id:??) landed as
https://crrev.com/8368b0bf31d5a9fe357c60de91f6c83b08b78267
Cr-Commit-Position: refs/heads/master@{#321440}