MouseEvent: mousedown - preventDefault() for text selection

### DOM Events

#### Test Description: MouseEvent - Text selection is disabled after cancelling mousedown event.

Use mouse to select the whole line here

Steps:

1. Make sure text in the above green box can be selected using mouse
2. Dismiss the selection, if any, by clicking at the green box with mouse
3. Now, click the button:
4. Drag mouse to select the whole line of the text inside the above green box
5. Click the button:

Test passes if the word "PASS" appears below after following the above steps.

Test result:
FAIL