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
[email protected]
NOTRY=true

Review URL: https://codereview.chromium.org/846213003

Cr-Commit-Position: refs/heads/master@{#311232}
15 files changed