Ignore setNeedsDisplay for SolidColorScrollbarLayer

Blink assumes that all scrollbar layers need repaint when scrolling.
However, SolidColorScrollbarLayer updates itself, without needing
to repaint. Ignore setNeedsDisplay for it.

BUG=333026

Review URL: https://codereview.chromium.org/189523002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@255720 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 255720