[SVG2] Add tabindex handling in svg.

LayoutTests/svg/custom/tabindex-order-expected.txt

Index: LayoutTests/svg/custom/tabindex-order-expected.txt
diff --git a/LayoutTests/svg/custom/tabindex-order-expected.txt b/LayoutTests/svg/custom/tabindex-order-expected.txt
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+++ b/LayoutTests/svg/custom/tabindex-order-expected.txt
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+This page tests that the SVG tabbing order is respected properly.
+
+To test, put focus in "a". Pressing Tab should focus "a" through "k" in order, and pressing Shift-Tab should reverse the order.
+
+ Tabbing forward....
+
+<rect id="a" tabindex="1"> focused
+<rect id="b" tabindex="1"> focused
+<rect id="c" tabindex="1"> focused
+<rect id="d" tabindex="1"> focused
+<rect id="e" tabindex="3"> focused
+<rect id="f" tabindex="4"> focused
+<rect id="g" tabindex="6"> focused
+<rect id="h" tabindex="6"> focused
+<rect id="i" tabindex="0"> focused
+<rect id="j" tabindex="0"> focused
+<rect id="k" tabindex="0"> focused
+
+Tabbing backward....
+
+<rect id="k" tabindex="0"> focused
+<rect id="j" tabindex="0"> focused
+<rect id="i" tabindex="0"> focused
+<rect id="h" tabindex="6"> focused
+<rect id="g" tabindex="6"> focused
+<rect id="f" tabindex="4"> focused
+<rect id="e" tabindex="3"> focused
+<rect id="d" tabindex="1"> focused
+<rect id="c" tabindex="1"> focused
+<rect id="b" tabindex="1"> focused
+<rect id="a" tabindex="1"> focused
+
+Test finished
+
+