Revert "Remove impl-side-painting from about:flags"

Revert "Remove impl-side-painting from about:flags"

This reverts commit a0b3c5b4bd504b6c699984149bd73ecf1fdc4e44.

Blink gardening. Looks this patch broke 2000+ layout tests again.
The previous revert was: https://codereview.chromium.org/846813002

From the dashboard, the root cause could be in the range, [r311167 : r311172].
http://test-results.appspot.com/dashboards/flakiness_dashboard.html#tests=virtual%2Fslimmingpaint%2Ffast%2Fmulticol%2Fcomposited-relpos-2nd-column.html%2Cvirtual%2Fslimmingpaint%2Ffast%2Fmulticol%2Fcomposited-relpos-overlapping-will-change.html%2Cvirtual%2Fslimmingpaint%2Ffast%2Fmulticol%2Fcomposited-with-child-layer-in-next-column.html%2Cvirtual%2Fslimmingpaint%2Ffast%2Fmulticol%2Fcomposited-opacity-2nd-and-3rd-column.html

r311170 is likely to a suspicious patch. Let me revert this and see what's going on.

BUG=169282
TBR=danakj@chromium.org
NOTRY=true

Committed: https://crrev.com/5e85a7826b825b006a2d1a50c391565a036bfe06
Cr-Commit-Position: refs/heads/master@{#311232}

[hayato, 2015-01-13 07:32:31]

The CQ bit was checked by hayato@chromium.org

[commit-bot: I haz the power, 2015-01-13 07:33:07]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/846213003/1

[commit-bot: I haz the power, 2015-01-13 07:34:46]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-01-13 07:36:13]

Patchset 1 (id:??) landed as
https://crrev.com/5e85a7826b825b006a2d1a50c391565a036bfe06
Cr-Commit-Position: refs/heads/master@{#311232}