Implement addPath() method for Path object

LayoutTests/fast/canvas/canvas-path-addpath-expected.txt

Index: LayoutTests/fast/canvas/canvas-path-addpath-expected.txt
diff --git a/LayoutTests/fast/canvas/canvas-path-addpath-expected.txt b/LayoutTests/fast/canvas/canvas-path-addpath-expected.txt
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+++ b/LayoutTests/fast/canvas/canvas-path-addpath-expected.txt
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+Test addPath() method.
+
+On success, you will see a series of "PASS" messages, followed by "TEST COMPLETE".
+
+
+Test addPath() with transform as identity matrix.
+PASS imgdata[4] is 255
+PASS imgdata[5] is 255
+PASS imgdata[6] is 0
+PASS imgdata[7] is 255
+
+Test addPath() with transform as translate(100, -100).
+PASS imgdata[4] is 255
+PASS imgdata[5] is 255
+PASS imgdata[6] is 0
+PASS imgdata[7] is 255
+
+Test addPath() with transform as null.
+PASS p1.addPath(p2, null) threw exception TypeError: Failed to execute 'addPath' on 'Path': parameter 2 is not of type 'SVGMatrix'..
+
+PASS successfullyParsed is true
+
+TEST COMPLETE
+