Add is_chromecast to GN build.

content/shell/BUILD.gn

Index: content/shell/BUILD.gn
diff --git a/content/shell/BUILD.gn b/content/shell/BUILD.gn
index 010758e5fd99e1f3937360e551b773b49ef32a41..c69e5968c47e8664bfcd378c7990feaa9474b85c 100644
--- a/content/shell/BUILD.gn
+++ b/content/shell/BUILD.gn
@@ -19,6 +19,8 @@ declare_args() {
   # something reasonably current; the "77.34.5" is a hint that this isn't a
   # standard Chrome.
   content_shell_version = "19.77.34.5"
+
+  use_custom_freetype = is_linux && !is_chromecast
 }
 
 static_library("content_shell_lib") {
@@ -314,7 +316,7 @@ static_library("content_shell_lib") {
     ]
   }
 
-  if (is_linux) {
+  if (use_custom_freetype) {
     deps += [ "//third_party/freetype2" ]
   }