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Pointer Capture was being rejected by chrome when an app came into
focus using keyboard shortcuts (Alt-Tab), but was working as intended
when an app came into focus using the mouse or touchscreen. It was being
rejected because the capture_surface was the sub-surface of the current
focused surface.

Since the client is responsible for providing the capture surface
through the wayland pointer constraints protocol, it is possible
for the client to request capture on a sub-surface of the current
active surface. Requesting capture on a sub-surface is a valid request
for capture. Rather than ensuring that the capture surface is focused
(used for directing key events), we verify that the active window
contains the window of the capture surface in its hierarchy before we
allow the pointer to be constrained to the surface.

This also allows the client to change the cursor while the client has
capture, even when the mouse cursor is not currently over a client
window.

BUG=b:153973515
TEST=manual: request pointer capture, leave app, return to app using
Alt+Tab, verify app has pointer capture.
TEST=exo_unittests

Change-Id: I56e76469f35ea43100748ebe27966464e32805fc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2559255Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Reviewed-by: Tetsui Ohkubo <tetsui@chromium.org>
Commit-Queue: Prabir Pradhan <prabirmsp@chromium.org>
Auto-Submit: Prabir Pradhan <prabirmsp@chromium.org>
Cr-Commit-Position: refs/heads/master@{#832613}