Keep observing to-be-snapped window when received a different type

If the WM_EVENT_SNAP_* is fired before ARC processes the previous
WMEvent (e.g. WM_EVENT_MAXIMIZE), ToBeSnappedWindowsObserver can receive
the state type change callback for the previous event first.
In that case, we shouldn’t stop observing the window state type, and
should keep waiting for the state type to change to a snapped one.

In general, we don’t expect that two WMEvents get triggered in such a
short period of time, but in theory it can and indeed it sometimes
happens in tast-test depending on the timing.

BUG=b:273141544
BUG=b:233559806
TEST=ClientControlledStateTestClamshellAndTablet.SnapBeforePreviousEventIsApplied

Change-Id: I09521345361d287fc45334c13e832419fb1f85cd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4340849
Reviewed-by: Xiaoqian Dai <xdai@chromium.org>
Commit-Queue: Toshiki Kikuchi <toshikikikuchi@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1117904}