cobalt/build: Build chromium_ios-arm64 with Blink (#7106)

Just setting `target_os="ios"` enables a WebKit-based build, which is
very different from most other Chromium ports and does not help with the
idea of getting more coverage for something similar to a tvOS build.

Bug: 443745131

Author: rakuco

cobalt/build/configs/chromium_ios-arm64/args.gn

@@ -1,2 +1,3 @@
 target_os = "ios"
 target_cpu = "arm64"
+use_blink = true