Mark WebInputEventBuilderMacTest.ScrollWheelMatchesUIEvent flaky

NOTREECHECKS=true
NOTRY=true
TBR=tapted@chromium.org,avi@chromium.org
BUG=640457

Review-Url: https://codereview.chromium.org/2286713002
Cr-Commit-Position: refs/heads/master@{#414702}

content/browser/renderer_host/input/web_input_event_builders_mac_unittest.mm

@@ -563,9 +563,10 @@ TEST(WebInputEventBuilderMacTest, DomKeyFlagsChanged) {
   }
 }
 
+// Flaky - https://crbug.com/640457
 // Test that a ui::Event and blink::WebInputEvent made from the same NSEvent
 // have the same values for comparable fields.
-TEST(WebInputEventBuilderMacTest, ScrollWheelMatchesUIEvent) {
+TEST(WebInputEventBuilderMacTest, DISABLED_ScrollWheelMatchesUIEvent) {
   CGFloat delta_x = 123;
   CGFloat delta_y = 321;
   NSPoint location = NSMakePoint(11, 22);