aura: changes when OnHostWill/DidProcessBoundsChange is called

WindowTreeHostPlatform::OnBoundsChanged() calls to OnHostWillProcessBoundsChange
on entering, and before exiting calls to OnHostDidProcessBoundsChange(). It turns
out WindowTreeHostPlatform::OnBoundsChanged() may be called recursively, which
confuses code in WebContentsViewAura that assumes this doesn't happen. Rather
than make observers track the depth of recursion, this makes
WindowTreeHostPlatform only notify on first entry/exit.

BUG=958449
TEST=covered by test

Change-Id: Ia039cb619457a7ad42768085900d7290761bbd0a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1593493
Commit-Queue: Scott Violet <sky@chromium.org>
Reviewed-by: Evan Stade <estade@chromium.org>
Cr-Commit-Position: refs/heads/master@{#656130}

diff --git a/ui/aura/BUILD.gn b/ui/aura/BUILD.gn
index d154a67f5..d12cac7 100644
--- a/ui/aura/BUILD.gn
+++ b/ui/aura/BUILD.gn

@@ -411,6 +411,7 @@
     "window_occlusion_change_builder_unittest.cc",
     "window_occlusion_tracker_unittest.cc",
     "window_targeter_unittest.cc",
+    "window_tree_host_platform_unittest.cc",
     "window_tree_host_unittest.cc",
     "window_unittest.cc",
   ]