NotificationViewMD: Validate after ToggleExpanded

NotificationViewMD::ButtonPressed() crashes when a notification popup is
expanded and that causes the popup to be removed. (It happens when
there's not enough screen space and the topmost notification is
expanded.)

This CL checks if |this| is still valid after ToggleExpanded by using
WeakPtr. Also it removes old workaround for the same issue that wasn't
working. (Added in https://crrev.com/114503004)

TEST=NotificationViewMD.TestDeleteOnToggleExpanded
BUG=918933

Change-Id: I9e1290112d6eaa2ab7155eb291764cd38e854f7e
Reviewed-on: https://chromium-review.googlesource.com/c/1400431
Commit-Queue: Tetsui Ohkubo <tetsui@chromium.org>
Reviewed-by: Evan Stade <estade@chromium.org>
Cr-Commit-Position: refs/heads/master@{#621850}
3 files changed