apple: Convert GN libs lists to frameworks in //skia.

GN recently added support for Apple frameworks to link, rather than
overloading the libs lists. This pulls .frameworks out of the libs
lists, so that GN can stop supporting .frameworks in libs in the
future.

Bug: 1052560
Change-Id: I334e4309d9d5dfe1a0fb45836925dddc532b4fde
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2277973
Commit-Queue: Robert Sesek <rsesek@chromium.org>
Commit-Queue: Mike Klein <mtklein@google.com>
Auto-Submit: Robert Sesek <rsesek@chromium.org>
Reviewed-by: Mike Klein <mtklein@google.com>
Cr-Commit-Position: refs/heads/master@{#784472}

skia/BUILD.gn

@@ -552,7 +552,7 @@ component("skia") {
   if (is_mac) {
     deps += [ ":skia_metal" ]
     defines += [ "SK_METAL" ]
-    libs = [
+    frameworks = [
       "AppKit.framework",
       "CoreFoundation.framework",
       "CoreGraphics.framework",
@@ -565,7 +565,7 @@ component("skia") {
     sources += skia_dawn_sources
   }
   if (is_ios) {
-    libs = [ "ImageIO.framework" ]
+    frameworks = [ "ImageIO.framework" ]
     set_sources_assignment_filter([])
     sources += [
       "//third_party/skia/src/utils/mac/SkCGBase.h",