Implement addPath() method for Path object

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

+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 non-invertible transform.
+PASS imgdata[4] is not 255
+PASS imgdata[5] is not 255
+PASS imgdata[6] is 0
+PASS imgdata[7] is not 255
+
+Test addPath() with transform as null or invalid type.
+PASS imgdata[4] is 255
+PASS imgdata[5] is 0
+PASS imgdata[6] is 0
+PASS imgdata[7] is 255
+
+Test addPath() with path as null and invalid type
+PASS p7.addPath(null, m) threw exception TypeMismatchError: Failed to execute 'addPath' on 'Path': The 1st argument provided is either null, or an invalid Path object..
+PASS p7.addPath([], m) threw exception TypeMismatchError: Failed to execute 'addPath' on 'Path': The 1st argument provided is either null, or an invalid Path object..
+PASS p7.addPath({}, m) threw exception TypeMismatchError: Failed to execute 'addPath' on 'Path': The 1st argument provided is either null, or an invalid Path object..
+
+PASS successfullyParsed is true
+
+TEST COMPLETE
+