Implement HTMLMarqueeElement's animation in private scripts

LayoutTests/fast/html/marquee-scroll.html

Index: LayoutTests/fast/html/marquee-scroll.html
diff --git a/LayoutTests/fast/html/marquee-scroll.html b/LayoutTests/fast/html/marquee-scroll.html
index 2c8d49f4573b4a617f541036a0cb42f537546447..d7555766ff9957417a8ae8f6873d53bad5a7d1b7 100644
--- a/LayoutTests/fast/html/marquee-scroll.html
+++ b/LayoutTests/fast/html/marquee-scroll.html
@@ -7,9 +7,13 @@ REGRESSION: scrolldelay=0 causes marquee not to display</p>
 
 <p>The marquee below is defined with a scrollDelay value of zero and with the truespeed attribute assigned to an empty string.  It should scroll until the text, &quot;The marquee scroll test passed&quot is visible and left justified.</p>
 
+<p>This test uses cyan characters to make the result image deterministic.
+(Otherwise, the result image changes depending on when the content_shell takes a snapshot.)
+In order to run this test manually, change the color to black.</p>
+
 <marquee width="50%" behavior="slide" bgcolor="cyan" truespeed="" scrollDelay="0">
-The marquee scroll test passed
+<span style="color:cyan">The marquee scroll test passed</span>
 </marquee>
 
 </body>
-</html>
+</html>