Introduce ScopedEventTargetingBlocker class.

It's a class to temporarily block event targeting for a window. There
can be multiple ScopedEventTargetingBlocker placed on the same window
and when that happens, the original event targeting policy will only be
restored after all ScopedEventTargetingBlockers are removed from the
window.

No functional change in this CL. This CL will be useful in my another CL
for Bug 997885.

Bug: 997885
Change-Id: I5f77eb001428be896570d5f3a5b92d98465374a2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1835279
Commit-Queue: Xiaoqian Dai <xdai@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
Cr-Commit-Position: refs/heads/master@{#703060}

• ash/home_screen/home_launcher_gesture_handler.cc[diff]
• ash/wm/overview/scoped_overview_transform_window.cc[diff]
• ash/wm/overview/scoped_overview_transform_window.h[diff]
• components/exo/client_controlled_shell_surface.cc[diff]
• ui/aura/BUILD.gn[diff]
• ui/aura/scoped_window_event_targeting_blocker.cc[Added - diff]
• ui/aura/scoped_window_event_targeting_blocker.h[Added - diff]
• ui/aura/window.cc[diff]
• ui/aura/window.h[diff]
• ui/aura/window_unittest.cc[diff]