Remove redundant disable-impl-side-painting flag

Remove redundant disable-impl-side-painting flag

After https://codereview.chromium.org/818233003 lands, then all layout
tests will explicitly either pass an enable or disable impl-side
painting flag.  This means that the content shell global disable flag
can be removed.

Once this has landed, then the whitelisted enable flags can also be
cleaned up in Blink, and the flag removed entirely.

BUG=381919
Committed: https://crrev.com/25e524f0c34abf359a51be3d63531d77e12b75ee
Cr-Commit-Position: refs/heads/master@{#312582}

[enne (OOO), 2015-01-21 00:33:49]

enne@chromium.org changed reviewers:
+ pfeldman@chromium.org

[chrishtr, 2015-01-21 00:36:07]

chrishtr@chromium.org changed reviewers:
+ chrishtr@chromium.org

[chrishtr, 2015-01-21 00:36:07]

The CQ bit was checked by chrishtr@chromium.org

[chrishtr, 2015-01-21 00:36:08]

lgtm

[commit-bot: I haz the power, 2015-01-21 00:37:39]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/774783003/100001

[enne (OOO), 2015-01-21 00:39:48]

The CQ bit was unchecked by enne@chromium.org

[enne (OOO), 2015-01-22 00:41:36]

enne@chromium.org changed reviewers:
+ mkwst@chromium.org

[enne (OOO), 2015-01-22 00:42:01]

+mkwst as alternate content/shell OWNER

[Mike West, 2015-01-22 06:57:47]

LGTM.

[Mike West, 2015-01-22 06:57:52]

The CQ bit was checked by mkwst@chromium.org

[commit-bot: I haz the power, 2015-01-22 06:58:57]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/774783003/100001

[commit-bot: I haz the power, 2015-01-22 07:45:37]

Committed patchset #6 (id:100001)

[commit-bot: I haz the power, 2015-01-22 07:46:54]

Patchset 6 (id:??) landed as
https://crrev.com/25e524f0c34abf359a51be3d63531d77e12b75ee
Cr-Commit-Position: refs/heads/master@{#312582}