Unprefix -webkit-filter and add a pixel test for it.

sky/tests/lowlevel/filters.sky

Index: sky/tests/lowlevel/filters.sky
diff --git a/sky/tests/harness/reftest-mismatch.sky b/sky/tests/lowlevel/filters.sky
similarity index 64%
copy from sky/tests/harness/reftest-mismatch.sky
copy to sky/tests/lowlevel/filters.sky
index 44bcc28f18cc38c3776918e5454f2fb0fbd28282..f08f6e927b913638b5907898e5c677b03bb952c3 100644
--- a/sky/tests/harness/reftest-mismatch.sky
+++ b/sky/tests/lowlevel/filters.sky
@@ -1,6 +1,8 @@
 <foo>
 <import src="../resources/run-after-display.sky" as="runAfterDisplay" />
-This is a mismatch reftest. Should not match.
+
+<div style="filter: grayscale(1) blur(2px); color: green;">This should be gray and blurred</div>
+
 <script>
 runAfterDisplay(function() {
     internals.notifyTestComplete("");