Set track_start on solid color layers to the margin.

Set track_start on solid color layers to the margin.

The scrollbar margin on the overlay scrollbar theme is used to distance
the thumb from the edge of the screen on all sides, not just the
widthwise side.  This was already computed properly in the theme's
TrackRect, but it wasn't communicated to the WebScrollbarLayer.

Also, shrink the margin to 3 DIPs on Android, which on closer inspection
better matches the system theme.

Chromium-side change at https://codereview.chromium.org/234603002/
NOTRY=true
BUG=361923
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=171404

[aelias_OOO_until_Jul13, 2014-04-11 04:35:14]

Hi, this is just a small change to better match Android system scrollbar style. 
tony@ for Source/core and Source/platform, abarth@ for new integer in public
API.

[abarth-chromium, 2014-04-11 05:49:30]

public/ LGTM

We don't need to stage the API change?

[aelias_OOO_until_Jul13, 2014-04-11 07:12:37]

We'll need to stage it.  It's just on the Chromium side this time that two
versions of the function will exist temporarily.

[tony, 2014-04-11 17:05:54]

LGTM

[aelias_OOO_until_Jul13, 2014-04-13 00:29:09]

The CQ bit was checked by aelias@chromium.org

[commit-bot: I haz the power, 2014-04-13 00:29:21]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/aelias@chromium.org/234613002/1

[commit-bot: I haz the power, 2014-04-13 00:29:45]

Change committed as 171404