Forward bundle product_type to the compile_xcassets tool.

tools/gn/function_toolchain.cc

Index: tools/gn/function_toolchain.cc
diff --git a/tools/gn/function_toolchain.cc b/tools/gn/function_toolchain.cc
index 69ca961cfdae0a5c1707ef7c45e3a8dd0ca6a34d..0072e65ddbafa3be4f3352f65fe230eb63ce80cf 100644
--- a/tools/gn/function_toolchain.cc
+++ b/tools/gn/function_toolchain.cc
@@ -317,7 +317,7 @@ const char kToolchain_Help[] =
     "  toolchain_args\n"
     "    Overrides for build arguments to pass to the toolchain when invoking\n"
     "    it. This is a variable of type \"scope\" where the variable names\n"
-    "    correspond to varibles in declare_args() blocks.\n"
+    "    correspond to variables in declare_args() blocks.\n"
     "\n"
     "    When you specify a target using an alternate toolchain, the master\n"
     "    build configuration file is re-interpreted in the context of that\n"
@@ -858,7 +858,17 @@ const char kTool_Help[] =
     "  copied.\n"
     "\n"
     "  The compile_xcassets tool will be called with one or more source (each\n"
-    "  an asset catalog) that needs to be compiled to a single output.\n"
+    "  an asset catalog) that needs to be compiled to a single output. The\n"
+    "  following substitutions are avaiable:\n"
+    "\n"
+    "    {{inputs}}\n"
+    "        Expands to the list of .xcassets to use as input to compile the\n"
+    "        asset catalog.\n"
+    "\n"
+    "    {{bundle_product_type}}\n"
+    "        Expands to the product_type of the bundle that will contain the\n"
+    "        compiled asset catalog. Usually corresponds to the product_type\n"
+    "        property of the corresponding create_bundle target.\n"
     "\n"
     "Separate linking and dependencies for shared libraries\n"
     "\n"