Fix incorrect dependency on "//cc" in "//skia".
Fix incorrect dependency on "//cc" in "//skia".

iOS cannot depends on //cc (nor //media) so put the dependency behind
an !is_ios condition.

Fix the following error when running "gn gen" for iOS after adding
//skia:skia_unittests to //BUILD.gn:

ERROR at //media/BUILD.gn:697:3: Assignment had no effect.
  configs += [ ":media_config" ]
  ^----------------------------
You set the variable "configs" here and it was unused before it went
out of scope.

Note that the error is the dependency on //media (which is not
supported on iOS).

BUG=459705

Review URL: https://codereview.chromium.org/1279453004

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#342595}

CapOM/ChromiumGStreamerBackend,ltilve/ChromiumGStreamerBackend,CapOM/ChromiumGStreamerBackend,CapOM/ChromiumGStreamerBackend,ltilve/ChromiumGStreamerBackend,ltilve/ChromiumGStreamerBackend,CapOM/ChromiumGStreamerBackend,CapOM/ChromiumGStreamerBackend,ltilve/ChromiumGStreamerBackend,CapOM/ChromiumGStreamerBackend,ltilve/ChromiumGStreamerBackend,CapOM/ChromiumGStreamerBackend,CapOM/ChromiumGStreamerBackend,ltilve/ChromiumGStreamerBackend,ltilve/ChromiumGStreamerBackend,ltilve/ChromiumGStreamerBackend,CapOM/ChromiumGStreamerBackend,ltilve/ChromiumGStreamerBackend