[SVG2] overflow should be ignored in img and background-image.

LayoutTests/svg/as-image/resources/overflow-hidden.svg

Index: LayoutTests/svg/as-image/resources/overflow-hidden.svg
diff --git a/LayoutTests/svg/as-image/resources/overflow-hidden.svg b/LayoutTests/svg/as-image/resources/overflow-hidden.svg
new file mode 100644
index 0000000000000000000000000000000000000000..78849311bcc29be874cfa3af9a734516dd6fb4be
--- /dev/null
+++ b/LayoutTests/svg/as-image/resources/overflow-hidden.svg
@@ -0,0 +1,13 @@
+<?xml version="1.0" encoding="utf-8"?>
+<!-- Generator: Adobe Illustrator 17.1.0, SVG Export Plug-In . SVG Version: 6.00 Build 0)  -->
+<!DOCTYPE svg PUBLIC "-//W3C//DTD SVG 1.1//EN" "http://www.w3.org/Graphics/SVG/1.1/DTD/svg11.dtd">
+<svg version="1.1" id="Layer_1" xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" x="0px" y="0px"
+	 width="40px" height="40px" viewBox="0 0 40 40" enable-background="new 0 0 40 40" xml:space="preserve" overflow="hidden">
+<linearGradient id="SVGID_1_" gradientUnits="userSpaceOnUse" x1="5.8579" y1="5.8579" x2="34.1421" y2="34.1421" gradientTransform="matrix(0.7071 0.7071 -0.7071 0.7071 20 -8.2843)">
+	<stop  offset="0" style="stop-color:#00A995"/>
+	<stop  offset="0" style="stop-color:#3C3795"/>
+	<stop  offset="0.5056" style="stop-color:#A53594"/>
+	<stop  offset="1" style="stop-color:#DA2128"/>
+</linearGradient>
+<polygon fill="url(#SVGID_1_)" points="20,40 0,20 20,0 40,20 "/>
+</svg>