Revert of Remove the acclerated overflow scroll and fixed root background flags.

Revert of Remove the acclerated overflow scroll and fixed root background flags. (patchset #1 of https://codereview.chromium.org/479673003/)

Reason for revert:
This seems to have broken most of the virtual/gpu/compositedscrolling/overflow. See http://build.chromium.org/p/chromium.webkit/builders/WebKit%20Linux/builds/35692 and http://build.chromium.org/p/chromium.webkit/builders/WebKit%20Mac10.9/builds/10128 for examples.

http://test-results.appspot.com/dashboards/flakiness_dashboard.html#group=%40ToT%20Blink&tests=virtual/gpu/compositedscrolling/overflow/content-gains-scrollbars.html,virtual/gpu/compositedscrolling/overflow/nested-render-surfaces-with-intervening-clip.html,virtual/gpu/compositedscrolling/overflow/nested-render-surfaces-with-rotation.html,virtual/gpu/compositedscrolling/overflow/nested-render-surfaces.html,virtual/gpu/compositedscrolling/overflow/overflow-auto-with-touch-toggle.html,virtual/gpu/compositedscrolling/overflow/overflow-auto-with-touch.html,virtual/gpu/compositedscrolling/overflow/overflow-overlay-with-touch.html,virtual/gpu/compositedscrolling/overflow/overflow-scrollbar-layers.html,virtual/gpu/compositedscrolling/scrollbars/overflow-scrollbar-combinations.html shows all the tests.

Original issue's description:
> Remove the acclerated overflow scroll and fixed root background flags.
> 
> These flags are merged into --{dis,en}able-prefer-compositing-to-lcd-text
> which controls these along with fixed position.
> 
> Depends on https://codereview.chromium.org/479653003/.
> 
> R=aelias, piman@chromium.org
> BUG=365851
> 
> Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=290218

TBR=piman@chromium.org,danakj@chromium.org
NOTREECHECKS=true
NOTRY=true
BUG=365851
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=290233

[Mike West, 2014-08-18 10:52:54]

Created Revert of Remove the acclerated overflow scroll and fixed root
background flags.

[commit-bot: I haz the power, 2014-08-18 10:53:15]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/mkwst@chromium.org/485663002/1

[commit-bot: I haz the power, 2014-08-18 10:54:21]

Committed patchset #1 (1) as 290233