Avoid a loop in writeString and writePad by zeroing padding first.

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
diff --git a/gyp/bench.gypi b/gyp/bench.gypi
index f809b64..e32db41 100644
--- a/gyp/bench.gypi
+++ b/gyp/bench.gypi
@@ -34,6 +34,7 @@
     '../bench/ShaderMaskBench.cpp',
     '../bench/TextBench.cpp',
     '../bench/VertBench.cpp',
+    '../bench/WriterBench.cpp',
   ],
 }