Fix web_test pointerevent_coordinates_when_locked on wpt

The test was failing on WPT[1] because test driver only takes integer
coordinates, but the test was using getBoundingClientRect which
returns float values.

This CL changes the test to set the "origin" for test driver actions
so that there is no need to use getBoundingClientRect.

[1] https://wpt.fyi/results/pointerevents/pointerlock/pointerevent_coordinates_when_locked.html

Change-Id: I56b2c052805b3bc174573564d176d6660accd7cb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1553645Reviewed-by: Lan Wei <lanwei@chromium.org>
Commit-Queue: Ella Ge <eirage@chromium.org>
Cr-Commit-Position: refs/heads/master@{#647877}

third_party/blink/web_tests/external/wpt/pointerevents/pointerlock/pointerevent_coordinates_when_locked.html

@@ -73,15 +73,15 @@
                 });
 
                 var actions = new test_driver.Actions();
-                actions.pointerMove(/* x = */ 0, /* y = */ 0, {origin: target}).pointerDown();
+                actions.pointerMove(/* x = */ 0, /* y = */ 0, {origin: div1}).pointerDown();
 
-                pos_x = target.getBoundingClientRect().x + target.offsetWidth / 2;
-                pos_y = target.getBoundingClientRect().y + target.offsetHeight / 2;
+                pos_x = div1.offsetWidth / 2;
+                pos_y = div1.offsetHeight / 2;
                 for (var i = 0; i < 10; i++) {
                     // Alternatively move left/right and up/down.
                     pos_x += ((-1)**i) * i * 10;
                     pos_y -= ((-1)**i) * i * 10;
-                    actions.pointerMove(pos_x, pos_y);
+                    actions.pointerMove(pos_x, pos_y, {origin: div1});
                 }
                 actions.pointerUp().send();
             }