Add test to track the number of hit tests on various events

LayoutTests/fast/events/hit-test-counts-expected.txt

+Count how many hit tests are required for various event scenarios. Hit tests can be expensive and it's often tempting to add more. These values should only ever be changed to go down, not up.
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+On success, you will see a series of "PASS" messages, followed by "TEST COMPLETE".
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+PASS successfullyParsed is true
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+TEST COMPLETE
+Event on a simple div
+---------------------
+Initial: 0
+MouseMove: 1
+MouseDown: 3
+MouseUp: 1
+Wheel: 1
+TouchStart: 1
+TouchMove: 0
+TouchEnd: 0
+GestureTapDown: 3
+GestureShowPress: 5
+GestureTap: 7
+GestureScrollBegin: 2
+GestureTapCancel: 2
+GestureScrollUpdate: 0
+GestureScrollEnd: 0
+
+Event over one iframe nested in another
+---------------------
+Initial: 0 0 0
+MouseMove: 1 1 1
+MouseDown: 2 2 3
+MouseUp: 1 1 1
+Wheel: 1 1 1
+TouchStart: 1 1 1
+TouchMove: 0 0 0
+TouchEnd: 0 0 0
+GestureTapDown: 5 5 5
+GestureShowPress: 7 7 7
+GestureTap: 3 3 7
+GestureScrollBegin: 4 4 4
+GestureTapCancel: 2 2 2
+GestureScrollUpdate: 0 0 0
+GestureScrollEnd: 0 0 0
+
+