Add debugging for view hierarchy modification during hit testing.

Introduces a boolean flag `is_hit_testing_` in `ViewAndroid`
to detect and log (via DumpWithoutCrashing) instances where
child views are removed while a hit test is in progress.

This helps diagnose potential UAFs or unexpected behavior caused
by concurrent modification of the view hierarchy during event
dispatch. `absl::Cleanup` ensures the flag is reset correctly.

Bug: 407571917
Change-Id: I4b2bb4da9eb1fd41f55909203e34ddc38792d932
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6489266
Commit-Queue: Aman Verma <amanvr@google.com>
Reviewed-by: Bo Liu <boliu@chromium.org>
Reviewed-by: Jonathan Ross <jonross@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1451352}