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To properly render the reticle, it needs to be drawn on the element it
is hitting, rather than last in the scene (otherwise, translucent
un-hittable elements may appear to have the reticle instead).  In the
future, we may transition to drawing the reticle last, always, for
simplicity.

However, if we don't do that, we need a way to have the reticle appear
properly on top of composite UI elements - elements made up of
sub-elements in a plane.  Without this change, something like a button
backplane may have the reticle, but portions of the reticle may appear
under a different button foreground element even though it's on the same
plane.  With this change, we traverse the hit element's subtree, and
draw the reticle after the last planar, visible child instead.  This
way, it's guaranteed to render on top of the composed element, unless
the element has hittable children floating in front.

BUG=
R=mthiesse, vollick

Cq-Include-Trybots: master.tryserver.chromium.android:android_optional_gpu_tests_rel;master.tryserver.chromium.linux:linux_optional_gpu_tests_rel;master.tryserver.chromium.mac:mac_optional_gpu_tests_rel;master.tryserver.chromium.win:win_optional_gpu_tests_rel
Change-Id: I33a1aef392901b74e3b5814338a4e4c5fb58eac9
Reviewed-on: https://chromium-review.googlesource.com/791151
Commit-Queue: Christopher Grant <cjgrant@chromium.org>
Reviewed-by: Biao She <bshe@chromium.org>
Reviewed-by: Ian Vollick <vollick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#519720}