Fix a crash on stroking empty paths with nv_path_rendering enabled

Fix the crash by defining that GrPathRenderer::drawPath and
GrPathRenderer::stencilPath are called only with non-empty paths.

Adds a new test "GpuDrawPath" and tests the condition.

BUG=1477
R=bsalomon@google.com

Author: kkinnunen@nvidia.com

Review URL: https://chromiumcodereview.appspot.com/22173002

git-svn-id: http://skia.googlecode.com/svn/trunk@10528 2bbb7eff-a529-9590-31e7-b0007b416f81
6 files changed