Bug 1534874 [wpt PR 15745] - Update wpt metadata, a=testonly

wpt-pr: 15745
wpt-type: metadata

testing/web-platform/meta/pointerlock/mouse_buttons_back_forward.html.ini

@@ -0,0 +1,6 @@
+[mouse_buttons_back_forward.html]
+  expected: TIMEOUT
+  max-asserts: 2
+  [Tests that when pointer is locked, the mouseup is preventable.]
+    expected: NOTRUN
+