x11/test: Use a custom-deleter for the XEvent in ScopedXI2Event.

BUG=none
R=sky@chromium.org

Previously committed in r248512, and reverted in r248526 because it introduced leaks.

Review URL: https://codereview.chromium.org/136153006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@248683 0039d316-1c4b-4281-b951-d872f2087c98
Makes WebTestThemeEngineMock draw scroll corner

This is needed for layout tests.

BUG=333499
TEST=none

Review URL: https://codereview.chromium.org/145973013

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@248684 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 248684