Add chromecast_build gyp variable and add rules when the variable is set to 1.

build/filename_rules.gypi

Index: build/filename_rules.gypi
diff --git a/build/filename_rules.gypi b/build/filename_rules.gypi
index 001b975c8768910352faddf92434d0db9fc10ed6..02837edddca6422e43ffbb72394064468cb6eb02 100644
--- a/build/filename_rules.gypi
+++ b/build/filename_rules.gypi
@@ -53,6 +53,11 @@
         ['exclude', '(^|/)chromeos/'],
       ],
     }],
+    ['chromecast==0', {
+      'sources/': [
+        ['exclude', '_(cast|chromecast)(_unittest)?\\.(h|cc)$'],

[jam, 2014/04/24 15:38:16]

since after the last meeting we're going to try to not add chromecast files as a
port, but instead use existing or new feature-specific flags, it seems this
isn't necessary anymore?

+      ],
+    }],
     ['>(nacl_untrusted_build)==0', {
       'sources/': [
         ['exclude', '_nacl(_unittest)?\\.(h|cc)$'],