Upgrade stepUp()/stepDown() implementation to match spec.

LayoutTests/fast/forms/time/time-stepup-stepdown-expected.txt

Index: LayoutTests/fast/forms/time/time-stepup-stepdown-expected.txt
diff --git a/LayoutTests/fast/forms/time/time-stepup-stepdown-expected.txt b/LayoutTests/fast/forms/time/time-stepup-stepdown-expected.txt
index 90fcd7b7f5e5efeea0db8b32a615a661f33682ef..e4fb7179ded5ab240705cdf07470c5ef3372ef64 100644
--- a/LayoutTests/fast/forms/time/time-stepup-stepdown-expected.txt
+++ b/LayoutTests/fast/forms/time/time-stepup-stepdown-expected.txt
@@ -1,11 +1,11 @@
-Check stepUp() and stepDown() bahevior for type=time.
+Check stepUp() and stepDown() behavior for type=time.
 
 On success, you will see a series of "PASS" messages, followed by "TEST COMPLETE".
 
 
 Invalid value
-PASS stepUp("", null, null) threw exception InvalidStateError: Failed to execute 'stepUp' on 'HTMLInputElement': The form element's current value is not a number..
-PASS stepDown("", null, null) threw exception InvalidStateError: Failed to execute 'stepDown' on 'HTMLInputElement': The form element's current value is not a number..
+PASS stepUp("", null, null) is "00:01"
+PASS stepDown("", null, null) is "00:00"
 Non-number arguments
 PASS stepUp("20:13", null, null, "0") is "20:13"
 PASS stepDown("20:13", null, null, "0") is "20:13"
@@ -24,12 +24,12 @@ Step=any
 PASS stepUp("20:13", "any", null) threw exception InvalidStateError: Failed to execute 'stepUp' on 'HTMLInputElement': This form element does not have an allowed value step..
 PASS stepDown("20:13", "any", null) threw exception InvalidStateError: Failed to execute 'stepDown' on 'HTMLInputElement': This form element does not have an allowed value step..
 Overflow/underflow
-PASS stepUp("20:13", "3.40282346e+38", null) threw exception InvalidStateError: Failed to execute 'stepUp' on 'HTMLInputElement': The form element's new value (3.40282346e+41) would be higher than the maximum (86399999), and snapping to the maximum would exceed the amount of acceptible error..
-PASS stepDown("20:13", "3.40282346e+38", null) threw exception InvalidStateError: Failed to execute 'stepDown' on 'HTMLInputElement': The form element's new value (-3.40282346e+41) would be lower than the minimum (0), and snapping to the minimum would exceed the amount of acceptible error..
-PASS stepUp("20:13", "1", "20:13") threw exception InvalidStateError: Failed to execute 'stepUp' on 'HTMLInputElement': The form element's new value (72781000) would be higher than the maximum (72780000), and snapping to the maximum would exceed the amount of acceptible error..
-PASS stepDown("20:13", "1", "20:13") threw exception InvalidStateError: Failed to execute 'stepDown' on 'HTMLInputElement': The form element's new value (72779000) would be lower than the minimum (72780000), and snapping to the minimum would exceed the amount of acceptible error..
-PASS stepUp("23:59", null, null) threw exception InvalidStateError: Failed to execute 'stepUp' on 'HTMLInputElement': The form element's new value (86400000) would be higher than the maximum (86399999), and snapping to the maximum would exceed the amount of acceptible error..
-PASS stepDown("00:00", null, null) threw exception InvalidStateError: Failed to execute 'stepDown' on 'HTMLInputElement': The form element's new value (-60000) would be lower than the minimum (0), and snapping to the minimum would exceed the amount of acceptible error..
+PASS stepUp("20:13", "3.40282346e+38", null) is "23:59:59.999"
+PASS stepDown("20:13", "3.40282346e+38", null) is "00:00:00"
+PASS stepUp("20:13", "1", "20:13") is "20:13:00"
+PASS stepDown("20:13", "1", "20:13") is "20:13:00"
+PASS stepUp("23:59", null, null) is "23:59"
+PASS stepDown("00:00", null, null) is "00:00"
 
 PASS successfullyParsed is true