Update pad-gesture-fling test to accommodate short-circuited fling

The pad-gesture-fling layout tests assumes the fling curve will generate a
minimum number of events.  This may not always hold, depending on the
fling curve parameters, the fling curve update frequency and whether the main
thread is slow.  Instead, impose a maximum limit on the number of fling updates
it takes to reach the minimum offset target, and use preventDefault() on the
mousewheel event to prevent the fling from terminating early.

Review URL: https://codereview.chromium.org/219243010

git-svn-id: svn://svn.chromium.org/blink/trunk@170597 bbb929c8-8fbe-4397-9dbb-9b2b20218538

third_party/WebKit/LayoutTests/fast/events/touch/gesture/pad-gesture-fling-expected.txt

@@ -6,8 +6,9 @@ On success, you will see a series of "PASS" messages, followed by "TEST COMPLETE
 PASS successfullyParsed is true
 
 TEST COMPLETE
-PASS cumulativeScrollX is >= 200
-PASS cumulativeScrollY is >= 200
+PASS actualWheelEventsOccurred is >= 2
+PASS cumulativeScrollX is >= 300
+PASS cumulativeScrollY is >= 300
 PASS successfullyParsed is true
 
 TEST COMPLETE

third_party/WebKit/LayoutTests/fast/events/touch/gesture/pad-gesture-fling.js

@@ -4,9 +4,9 @@ var actualWheelEventsOccurred = 0;
 var cumulativeScrollX = 0;
 var cumulativeScrollY = 0;
 
-var minimumWheelEventsExpected = 40;
-var minimumScrollXExpected = "200";
-var minimumScrollYExpected = "200";
+var minimumWheelEventsExpected = "2";
+var minimumScrollXExpected = 300;
+var minimumScrollYExpected = 300;
 
 var positionX = 10;
 var positionY = 11;
@@ -25,14 +25,17 @@ function recordWheelEvent(event)
     cumulativeScrollX += event.wheelDeltaX;
     cumulativeScrollY += event.wheelDeltaY;
 
-    if (actualWheelEventsOccurred == minimumWheelEventsExpected) {
-      shouldBeGreaterThanOrEqual('cumulativeScrollX', minimumScrollXExpected);
-      shouldBeGreaterThanOrEqual('cumulativeScrollY', minimumScrollYExpected);
+    if (cumulativeScrollX >= minimumScrollXExpected
+            && cumulativeScrollY >= minimumScrollYExpected) {
+      shouldBeGreaterThanOrEqual('actualWheelEventsOccurred', minimumWheelEventsExpected);
+      shouldBeGreaterThanOrEqual('cumulativeScrollX', minimumScrollXExpected.toString());
+      shouldBeGreaterThanOrEqual('cumulativeScrollY', minimumScrollYExpected.toString());
 
       isSuccessfullyParsed();
       if (window.testRunner)
           testRunner.notifyDone();
     }
+    event.preventDefault();
 }
 
 document.addEventListener("mousewheel", recordWheelEvent);