Make Chromecast build use the built-in cvox extension

kChromeVoxExtensionId should match the built-in extension
id, not the web store version.  Added rule to set
IS_CHROMECAST define.

Bug: b/73383411
Test: None
Change-Id: I4d5da20066a54b2dd12a1f76cabc3214836117c0
Reviewed-on: https://chromium-review.googlesource.com/1087171Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Commit-Queue: Randy Rossi <rmrossi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#566006}

extensions/common/BUILD.gn

@@ -11,6 +11,10 @@ import("//mojo/public/tools/bindings/mojom.gni")
 # TODO(crbug.com/731689): Assert that extensions are enabled.
 
 source_set("common_constants") {
+  if (is_chromecast) {
+    defines = [ "IS_CHROMECAST" ]
+  }
+
   sources = [
     "constants.cc",
     "constants.h",

extensions/common/constants.cc

@@ -94,7 +94,7 @@ const int64_t kInvalidServiceWorkerVersionId = -1;
 
 namespace extension_misc {
 
-#if defined(OS_CHROMEOS)
+#if defined(OS_CHROMEOS) || defined(IS_CHROMECAST)
 // The extension id for the built-in component extension.
 const char kChromeVoxExtensionId[] = "mndnfokpggljbaajbnioimlmbfngpief";
 #else