Add enable_vr_shell build time flag and show icon in Daydream app luncher

chrome/android/BUILD.gn

Index: chrome/android/BUILD.gn
diff --git a/chrome/android/BUILD.gn b/chrome/android/BUILD.gn
index da3e6882753d2f7173ce61f2f3b6f566b9d642d1..3a81e2dcece7bfb09e3734f0e59ed02ed4d9e7aa 100644
--- a/chrome/android/BUILD.gn
+++ b/chrome/android/BUILD.gn
@@ -46,6 +46,9 @@ jinja_template("chrome_public_apk_manifest") {
     "min_sdk_version=16",
     "target_sdk_version=23",
   ]
+  if (enable_vr_shell) {
+    variables += [ "enable_vr_shell=1" ]
+  }
 }
 
 # GYP: //chrome/android/chrome_apk.gyp:chrome_sync_shell_apk_manifest