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

chrome/common/features.gni

Index: chrome/common/features.gni
diff --git a/chrome/common/features.gni b/chrome/common/features.gni
index 5ecfc4ca02f8f7fe37dc9ccf8782223f726eb433..8710824c4af8ba4e63894638af205e246359586f 100644
--- a/chrome/common/features.gni
+++ b/chrome/common/features.gni
@@ -31,6 +31,9 @@ declare_args() {
   # Set to true to bundle all the mash related mojo services into chrome.
   # Specify --mash to chrome to have chrome start the mash environment.
   enable_package_mash_services = is_chromeos
+
+  # Enables vr shell.
+  enable_vr_shell = false && is_android

[Ted C, 2016/08/02 21:38:11]

umm...while my gn foo is quite weak, this looks particularly odd to me.  why
isn't it just false?

[bshe, 2016/08/02 22:26:59]

Good question. I was trying to restrice other platforms use this build flag as
we only support Android initially. And I think I can mix default value with
value of "is_android". "gn args --list out/Debug | grep vr_shell" shows that the
default value is "false" without "is_android" included. So I imagine when
processing it, false is replaced with the value we set in gn args first and then
evaluated here. But I am not 100% sure
Anyway, I agree this is not very readable and we dont use it outside of Android
anyway. So remove "is_android" here. 

 }
 
 chrome_grit_defines = [