Revert "[iOS] Defaults to use system libjpeg on iOS."

Fix the configuration of libjpeg_turbo to build on iOS by using the
same settings as Mac. This is not used by Chrome on iOS AFAIK, but
appears to be used by WebRTC.

BUG=602682

Review URL: https://codereview.chromium.org/1886443002

Cr-Original-Commit-Position: refs/heads/master@{#386941}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: cb3f4114b1b74302570e17e09b59b097d6ee406b

secondary/third_party/libjpeg_turbo/BUILD.gn

@@ -82,7 +82,7 @@ if (current_cpu == "x86" || current_cpu == "x64") {
       } else {
         defines += [ "WIN64" ]
       }
-    } else if (is_mac) {
+    } else if (is_mac || is_ios) {
       defines += [ "MACHO" ]
       include_dirs = [ "mac" ]
     } else if (is_linux || is_android) {