Add 9 patch scrollbar assets to native theme

Add 9 patch scrollbar assets to native theme

9-patch scrollbar is a prettified version of overlay scrollbar that is
going to be used on aura. Add these assets with their ID to native theme
 aura. Add logic to use 9 patch scroll bar assets when overlay scrollbar
 is enabled. Also overlay scrollbar should only be painting thumbs, thus
 when overlay scrollbar is turned on native theme shouldn't need to
paint track and scrollbar corner.

This set of assets for 9 patch scrollbar intent to control stroke and
fill separately, while stroke does not provide asset for center of
9 patch. Different state is achieved by adjusting alpha of stroke and
fill separately.

BUG=307578

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=256930

Review URL: https://codereview.chromium.org/183973008

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@257744 0039d316-1c4b-4281-b951-d872f2087c98