Allow mouse drag from inside a <popup> to outside

With this CL, mousing-down inside a <popup> and dragging to outside
that <popup> will no longer light-dismiss the popup. This also changes
the trigger for light dismiss from "click" to "mousedown", which
mirrors platform conventions.

Bug: 1168738
Change-Id: I23aa035107409b9fb2f36ee90ef8a3c08d8e0c62
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2890150
Commit-Queue: Mason Freed <masonf@chromium.org>
Auto-Submit: Mason Freed <masonf@chromium.org>
Reviewed-by: Ionel Popescu <iopopesc@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#882881}

html/semantics/interactive-elements/the-popup-element/popup-light-dismiss.tentative.html

@@ -159,13 +159,14 @@
     },'An invoking element that was not used to invoke the popup should NOT be part of the ancestor chain');
 
     popup1.show();
-    popup2.show();
+    popup2.show(); // Popup1 is an ancestral element for popup2.
     assert_true(popup1.open);
     assert_true(popup2.open);
     const drag_actions = new test_driver.Actions();
-    await drag_actions.pointerMove(0,0,{origin: popup1})
+    // Drag *from* popup2 *to* popup1 (its ancestor).
+    await drag_actions.pointerMove(0,0,{origin: popup2})
       .pointerDown({button: drag_actions.ButtonType.LEFT})
-      .pointerMove(0,0,{origin: popup2})
+      .pointerMove(0,0,{origin: popup1})
       .pointerUp({button: drag_actions.ButtonType.LEFT})
       .send();
     test(t => {