[Cast Convergence] Remove is_chromecast in ANGLE.

is_chromecast is going to be removed from Chromium.  This is_chromecast
with is_castos, which will be set for builds that target Nest Linux
devices.

Upstream bug: https://crbug.com/1337084
Umbrella bug: https://crbug.com/1219802
Design doc (internal only): https://go/unified-cast-web-runtime

Bug: angleproject:7521
Change-Id: Id1a0df72e37868eabd1f851123eec615f6657570
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3782573
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Geoff Lang <geofflang@chromium.org>

gni/angle.gni

@@ -37,16 +37,16 @@ if (angle_has_build) {
     import("//build/config/win/visual_studio_version.gni")
   }
 
-  if (is_chromecast || ozone_platform_headless) {
+  if (is_castos || ozone_platform_headless) {
     angle_vulkan_display_mode = "headless"
   }
 
   declare_args() {
     angle_use_gbm = ozone_platform_gbm
-    angle_use_x11 = ozone_platform_x11 && !is_ggp &&
-                    (is_linux || is_chromeos) && !is_chromecast
+    angle_use_x11 =
+        ozone_platform_x11 && !is_ggp && (is_linux || is_chromeos) && !is_castos
     angle_use_wayland =
-        ozone_platform_wayland && !is_ggp && is_linux && !is_chromecast
+        ozone_platform_wayland && !is_ggp && is_linux && !is_castos
     angle_use_vulkan_display = (is_linux || is_chromeos) && !is_ggp
   }
 } else {