[views-ax] Serialize full tree immediately on Enable

As pointed out in a comment on CL:7004102, we should serialize the
full views-based AXTree immediately when we enable the kNativeAPIs mode.
Previously, serialization was postponed after execution of the current
task on the main thread. That could lead to a different behavior than
what it is today, and could cause issues with various assistive
technologies.

This CL replaces `SchedulePendingUpdate()` by `SendPendingUpdate()`.

Bug: 40672441
Change-Id: I1dddbcce1d509b1318abf8a0e42fe8f6159c8f50
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7046377
Reviewed-by: Javier Contreras <javiercon@microsoft.com>
Auto-Submit: Benjamin Beaudry <benjamin.beaudry@microsoft.com>
Commit-Queue: Benjamin Beaudry <benjamin.beaudry@microsoft.com>
Cr-Commit-Position: refs/heads/main@{#1530296}