Fix geometry mojo build dependencies

Everything using Rect in mojom files will indirectly need
to include //ui/gfx/geometry/mojo/geometry_struct_traits.h"
and for that to be allowed in gn, its build target
(//ui/gfx/geometry/mojo:struct_traits) needs to be a public
dependency.

This fixes about 200 dependency errors as reported by a gn
that can check dependencies in generated files (gn:57).

Bug: 938893
Change-Id: Ifbe6d0c6a0bc26e158badf751322509e66093252
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1508467Reviewed-by: Robert Sesek <rsesek@chromium.org>
Commit-Queue: Daniel Bratell <bratell@opera.com>
Cr-Commit-Position: refs/heads/master@{#638746}

ui/gfx/geometry/mojo/geometry.typemap

@@ -18,7 +18,7 @@ public_headers = [
   "//ui/gfx/geometry/vector3d_f.h",
 ]
 traits_headers = [ "//ui/gfx/geometry/mojo/geometry_struct_traits.h" ]
-deps = [
+public_deps = [
   "//ui/gfx/geometry/mojo:struct_traits",
 ]
 type_mappings = [