Remove "ForcedLayer" from Sky

Remove "ForcedLayer" from Sky

No one uses this mechanism.

TBR=esprehn@chromium.org

Committed: https://chromium.googlesource.com/external/mojo/+/26e4278bd177d7a5313024306a777e664fff5b11

[abarth-chromium, 2015-01-27 23:22:19]

Committed patchset #1 (id:1) manually as
26e4278bd177d7a5313024306a777e664fff5b11 (presubmit successful).

[esprehn, 2015-01-27 23:23:00]

lgtm, I wonder if we can get rid of OverflowClipLayer too. That'll probably
change the paint order some, but once we kill the paint phases I suspect it
won't matter.

[abarth-chromium, 2015-01-27 23:25:06]

Does the system know how to clip without OverflowClipLayer?

[esprehn, 2015-01-27 23:27:13]

On 2015/01/27 at 23:25:06, abarth wrote:
> Does the system know how to clip without OverflowClipLayer?

Yup, otherwise position: absolute + overflow: clip wouldn't work since that gets
you a NormalLayer. OverflowClipLayer is a "hack" to give you a layer, but not
mess with your paint order in the paint phases like NormalLayer did.

[abarth-chromium, 2015-01-27 23:29:27]

On 2015/01/27 at 23:27:13, esprehn wrote:
> On 2015/01/27 at 23:25:06, abarth wrote:
> > Does the system know how to clip without OverflowClipLayer?
> 
> Yup, otherwise position: absolute + overflow: clip wouldn't work since that
gets you a NormalLayer. OverflowClipLayer is a "hack" to give you a layer, but
not mess with your paint order in the paint phases like NormalLayer did.

Oh, you're saying use NormalLayer for clipping.  Yeah, that might work.