iOS: Convert GN libs lists to frameworks in //testing/iossim.

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: I54034bd08040e8aba6bd6d21e53a920fe42f3fad
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2277965
Auto-Submit: Robert Sesek <rsesek@chromium.org>
Reviewed-by: Rohit Rao <rohitrao@chromium.org>
Commit-Queue: Rohit Rao <rohitrao@chromium.org>
Commit-Queue: Robert Sesek <rsesek@chromium.org>
Cr-Commit-Position: refs/heads/master@{#784401}

testing/iossim/BUILD.gn

@@ -8,7 +8,7 @@ import("//build/config/mac/mac_sdk.gni")
 if (current_toolchain == host_toolchain) {
   executable("iossim") {
     sources = [ "iossim.mm" ]
-    libs = [ "Foundation.framework" ]
+    frameworks = [ "Foundation.framework" ]
   }
 } else if (current_toolchain == default_toolchain) {
   copy("iossim") {