[rust png] Restrict `libpng` visibility to `libwebp:cwebp` exe.

The `libwebp` **library** doesn't depend on `libpng`.  This CL makes
this more explicit, by tweaking visibilities of GN rules that cover
`libwebp` and `libpng` libraries.  In particular, this CL:

* Only allows depending on `libpng` from `libwebp:imagedec`.  This
  consequently disallows depending on `libpng` from other parts of
  `libwebp` (e.g. from the `libwebp` library).  This consequence is
  the main motivation/goal for this CL.
* Only allows depending on `libwebp:imagedec` from `libwebp:cwebp`
  (the latter is an `executable` target)
* Only allows depending on `libwebp:cwebp` from specific directories
  related to Android builds
* Deletes the `libwebp:dwebp` `executable` target (which was not used
  elsewhere in Chromium).  And consequently delete the
  `libwebp:imageenc` target which was only used from `libwebp:dwebp`.

Bug: 443128323
Change-Id: I28dc6553593c6db6e9317d3dd7e828ca093cf3ed
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6977689
Reviewed-by: James Zern <jzern@google.com>
Auto-Submit: Łukasz Anforowicz <lukasza@chromium.org>
Reviewed-by: Rick Byers <rbyers@chromium.org>
Commit-Queue: Łukasz Anforowicz <lukasza@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1523731}
NOKEYCHECK=True
GitOrigin-RevId: 269786b942d89b7695fbb89550fc191d1f2079d4

visibility.gni

diff --git a/visibility.gni b/visibility.gni
index 00d19fa..b6546f5 100644
--- a/visibility.gni
+++ b/visibility.gni
@@ -1,4 +1,5 @@
 # This list tracks all current users of libpng.
+#
 # libpng is actively being migrated, and calls to libpng should instead go
 # through Skia's `SkPngRustDecoder`. No new entries should be added to this
 # list.
@@ -29,10 +30,22 @@
 
   # Other dependency edges that seem like Chrome => ... => libpng dependencies
   # but really aren't.  TODO: Clarify this comment in follow-up CLs.
-  "//third_party/libwebp:*",
   "//third_party/weston/*",
 ]
 
+# Dependencies that do not really flow into Chrome binaries.
+visibility += [
+  # `build/config/android/*.gni` =>
+  #     => `libwebp:cwebp` (`executable`)
+  #     => `libwebp:imagedec`
+  #     => `libpng` dependency.
+  # The `cwebp` `executable` is used during build process, but depending on it
+  # does not result in linking the `libpng` library into Chrome binary.  See
+  # also `cwebp_exe_allowed_visibility` in
+  # `//third_party/libwebp/visibility.gni`.
+  "//third_party/libwebp:imagedec",
+]
+
 # Test/tool-only dependencies under `//third_party/angle`
 # (migrating these to `libpng_testonly_visibility` would require editing
 # ANGLE's `BUILD.gn` files to depend on the `libpng_for_testonly` target).