[VizHitTesting] Fix an unchecked access to a optional value

target_location has a value only if the target exists. When it doesn't
we early exit the DispatchMouseEvent(). In addition, we should skip
setting the mouse_down_post_transformed_coordinate_.

Bug: 985692
Change-Id: I4cd4dad3b1caefaf2bd7e81660e257a4bdb5fdfa
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1715227
Commit-Queue: Yi Gu <yigu@chromium.org>
Reviewed-by: Ken Buchanan <kenrb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#680119}