[SkiaBenchmarkingExtension] Handle empty pictures gracefully.

Skia's SkDebugCanvas::drawTo() asserts that the command vector is not
empty. We should catch this condition in Rasterize() to avoid crashing
on debug builds.

BUG=(trace-viewer:493)
R=piman@chromium.org, nduca, piman

Review URL: https://codereview.chromium.org/47913005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@232236 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 232236