Add SetScale to PPB_WidgetDev, use scale for painting

Add SetScale to PPB_WidgetDev, use scale for painting

In order to allow for HiDPI-aware scrollbars, this patch allows the plugin to
specify a scale factor used during widget paint operations.

The widgets (of which Scrollbar is the only current instance) will continue to
use the DIP coordinate system for events and sizing, which is consistent with
their WebKit implementation.

This change should maintain backward compat. with existing users of interface
'PPB_Wdiget(Dev);0.3'

BUG=140833
TEST=Run pre-built (with 0.3 interfaces) PDF plugin and verify scrollbars


Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=152495

Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=152889

[Josh Horwich, 2012-08-20 22:28:59]

Please take a look.

[brettw, 2012-08-20 23:31:13]

lgtm

[commit-bot: I haz the power, 2012-08-21 00:34:09]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/jhorwich@chromium.org/10824386/1

[commit-bot: I haz the power, 2012-08-21 02:51:46]

Change committed as 152495

[commit-bot: I haz the power, 2012-08-22 22:40:34]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/jhorwich@chromium.org/10824386/1

[commit-bot: I haz the power, 2012-08-23 00:47:56]

Change committed as 152889