[Shadergraph] Updated Blackboard Resizer styles

[esmelusina]

This change is in conjunction with this one in trunk https://github.cds.internal.unity3d.com/unity/unity/pull/1415, though pushing this now doesn't cause any problems.

I pulled hardcoded styling information for the 'resizer' that the blackboard uses and put them into the default styles so that they can be overridden accordingly. This is the package local override for those settings to prevent UI elements from fighting.

https://fogbugz.unity3d.com/f/cases/1253962/
https://fogbugz.unity3d.com/f/cases/1163697/

Manual testing-- the resize handle no longer blocks the scrollbars.

Yamato: (Select your branch):
https://yamato.prd.cds.internal.unity3d.com/jobs/902-Graphics

[ghost]

The resizer section is really hard to grab now. It feels like there's only around 3 pixels high where the mouse can grab it (even though the element is supposedly 6px tall). I think 10px by 10px would work a lot better.
Edit: the problem seems to be when the mouse approaches from the bottom. There's a space where the mouse should be turning into a resizer icon but it doesn't until you get around 3 pixels into the ui element. If you changed it to 10 px high I think this would fix that problem.

[esmelusina]

The resizer section is really hard to grab now. It feels like there's only around 3 pixels high where the mouse can grab it (even though the element is supposedly 6px tall). I think 10px by 10px would work a lot better.
Edit: the problem seems to be when the mouse approaches from the bottom. There's a space where the mouse should be turning into a resizer icon but it doesn't until you get around 3 pixels into the ui element. If you changed it to 10 px high I think this would fix that problem.

This resolves the issue after the trunk PR lands, but doesn't do much without that landing. I messed up or wasn't aware of the PR process in ono so it was a bit delayed.

[ghost]

New resizer handle is way easier to click now. Approving.