Fix the rendering error of SkDraw::drawVertices in gpu path for solid color.

If both textures and vertex-colors are NULL, drawVertices should stroke hairlines with the paint's color.
This behavior is a useful debugging mode to visualize the mesh.

BUG=skia:2266
R=bsalomon@google.com, reed@google.com

Author: yunchao.he@intel.com

Review URL: https://codereview.chromium.org/189963004

git-svn-id: http://skia.googlecode.com/svn/trunk/include@14985 2bbb7eff-a529-9590-31e7-b0007b416f81