Fix //third_party/libwebp dependencies.

This allows targets depending on //third_party/libwebp (as a group target)
to include headers (by marking the public headers of each subtarget as public
headers of the group), including those from the basic :libwebp_webp source set.

This resolves a couple of gn check issues in //third_party/blink/renderer/platform.

Change-Id: I66533ff7b1b1663dd6dce2842cef315e4ec866d8
Reviewed-on: https://chromium-review.googlesource.com/c/1329549Reviewed-by: James Zern <jzern@google.com>
Commit-Queue: Jeremy Roman <jbroman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#608570}

third_party/libwebp/BUILD.gn

@@ -389,6 +389,7 @@ group("libwebp") {
     ":libwebp_mux",
     ":libwebp_utils",
   ]
+  public_deps = [ ":libwebp_webp" ]
   public_configs = [ ":libwebp_config" ]
   if (use_dsp_neon) {
     deps += [ ":libwebp_dsp_neon" ]