Add to GN ppapi build.

ppapi/BUILD.gn

Index: ppapi/BUILD.gn
diff --git a/ppapi/BUILD.gn b/ppapi/BUILD.gn
index 78d5ce9aba06c9ca90c2160c0b0c49b6c12de1cc..77ad58cb019039a5e2e5f9eb0feab26d1f13deaa 100644
--- a/ppapi/BUILD.gn
+++ b/ppapi/BUILD.gn
@@ -716,4 +716,42 @@ component("ppapi_proxy") {
   }
 }
 
+component("ppapi_host") {
+  sources = [
+    "host/dispatch_host_message.h",
+    "host/error_conversion.cc",
+    "host/error_conversion.h",
+    "host/host_factory.h",
+    "host/host_message_context.cc",
+    "host/host_message_context.h",
+    "host/instance_message_filter.cc",
+    "host/instance_message_filter.h",
+    "host/message_filter_host.cc",
+    "host/message_filter_host.h",
+    "host/ppapi_host.cc",
+    "host/ppapi_host.h",
+    "host/ppapi_host_export.h",
+    "host/resource_host.cc",
+    "host/resource_host.h",
+    "host/resource_message_filter.cc",
+    "host/resource_message_filter.h",
+    "host/resource_message_handler.cc",
+    "host/resource_message_handler.h",
+  ]
+
+  defines = [ "PPAPI_HOST_IMPLEMENTATION" ]
+
+  deps = [
+    ":ppapi_c",
+    ":ppapi_ipc",
+    ":ppapi_proxy",
+    ":ppapi_shared",
+    "//base",
+    "//ipc",
+    "//ui/surface",
+    "//url",
+    #'../media/media.gyp:shared_memory_support',  TODO(GYP)
+  ]
+}
+
 # TODO(GYP) other targets from ppapi_tests.gyp