[accessibility] Smart pointer for AXPlatformNode

Introduce AXPlatformNode::Pointer, a type of std::unique_ptr<> for
owners of platform nodes. The node's `Destroy()` method is called to
initiate destruction. Migrate all owners of platform nodes to using this
new smart pointer rather than raw_ptr<> and manual calls to instances'
`Destroy()` method.

Also fixes memory leaks in various AccessibilityBridgeFuchsiaTest tests,
which were missing calls to `Destroy()`.

AX-Relnotes: n/a.
Bug: none
Change-Id: I3b7ab5e14726c098350a41126ee0bc26ce1aeef7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6298029
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Aaron Leventhal <aleventhal@chromium.org>
Commit-Queue: Kinuko Yasuda <kinuko@chromium.org>
Auto-Submit: Greg Thompson <grt@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1425038}