Allow blink for iOS to include libjpeg_turbo

A new build setting has been added that denotes whether blink
is being compiled, use that instead of is_ios.

Bug: 1411704
Change-Id: Ib41f6e54ea477b49f0fc1233cb3fba51196ce8e2
diff --git a/BUILD.gn b/BUILD.gn
index 5acf523..524890a 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -5,12 +5,13 @@
 # Do not use the targets in this file unless you need a certain libjpeg
 # implementation. Use the meta target //third_party:jpeg instead.
 
+import("//build/config/features.gni")
 import("//build/config/sanitizers/sanitizers.gni")
 if (current_cpu == "arm" || current_cpu == "arm64") {
   import("//build/config/arm.gni")
 }
 
-assert(!is_ios, "This is not used on iOS, don't drag it in unintentionally")
+assert(use_blink, "This is not used on if blink is not enabled, don't drag it in unintentionally")
 
 source_set("libjpeg_headers") {
   sources = [