Also add a benchmark to time the new improved writeString. Before
my change the bench took ~1.23ms and afterwards it takes ~.95ms.
Add some testing to ensure that writePad works properly.
TEST=Writer32Test, WriterBench
Review URL: https://codereview.appspot.com/6438045
git-svn-id: http://skia.googlecode.com/svn/trunk@4742 2bbb7eff-a529-9590-31e7-b0007b416f81