Move cast_is_debug to //chromecast/chromecast.gni

BUG=internal b/29257163
TEST=none

Review-Url: https://codereview.chromium.org/2214463002
Cr-Commit-Position: refs/heads/master@{#409851}

diff --git a/chromecast/chromecast.gni b/chromecast/chromecast.gni
index b327f79d..9e5a6ba 100644
--- a/chromecast/chromecast.gni
+++ b/chromecast/chromecast.gni

@@ -17,6 +17,11 @@
   # than any value the builder may assign to prevent attempted automatic updates
   # when the default value is used.
   cast_build_incremental = "999999"
+
+  # If true, IS_CAST_DEBUG_BUILD() will evaluate to 1 in version.h. Otherwise,
+  # it will evaluate to 0. Overriding this when is_debug=false is useful for
+  # doing engineering builds.
+  cast_is_debug = is_debug
 }
 
 declare_args() {