Remove 'FAST_MOBILE_SCROLLING'

Remove feature that is no longer used by blink as the Android port now turns fixed backgrounds into layers rendering this code obsolete.
    
BUG=none

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=161821

[atreat, 2013-11-06 17:46:11]

As discussed on IRC this patch turns FAST_MOBILE_SCROLLING into a runtime
enabled feature.

[atreat, 2013-11-06 17:47:04]

Adding Peter and Laszlo who might be interested.

[atreat, 2013-11-06 17:47:52]

This feature is turned on by android via a chromium CL:
https://codereview.chromium.org/62413002/

[Peter Beverloo, 2013-11-07 14:05:17]

+Sami

We turn backgrounds with background-attachment:fixed into layers, which make
them work just fine on Android.  I believe we can instead remove this code
altogether.

An example page made by Sami can be seen here:
http://jsbin.com/ULoSevu/1/quiet

[atreat, 2013-11-11 19:25:26]

New version that removes this compile flag altogether as it is unnecessary since
Android uses layers for painting this.

[abarth-chromium, 2013-11-11 21:50:34]

Great!  LGTM

[Peter Beverloo, 2013-11-12 13:47:44]

lgtm, thank you!

[commit-bot: I haz the power, 2013-11-12 15:10:51]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/adam.treat@samsung.com/59763011/60001

[commit-bot: I haz the power, 2013-11-12 16:14:46]

Change committed as 161821