Fix sandbox target in Windows GN build and enable it.

Review URL: https://codereview.chromium.org/804993003

Cr-Commit-Position: refs/heads/master@{#308471}

BUILD.gn

@@ -340,7 +340,6 @@ group("root") {
       "//ppapi:ppapi_c",
       "//printing",
       "//remoting/client/plugin",
-      "//sandbox",
       "//storage/browser",
       "//sync",
       "//third_party/WebKit/public:all_blink",

sandbox/win/BUILD.gn

@@ -65,6 +65,12 @@ source_set("sandbox") {
     "src/policy_target.h",
     "src/process_mitigations.cc",
     "src/process_mitigations.h",
+    "src/process_mitigations_win32k_dispatcher.cc",
+    "src/process_mitigations_win32k_dispatcher.h",
+    "src/process_mitigations_win32k_interception.cc",
+    "src/process_mitigations_win32k_interception.h",
+    "src/process_mitigations_win32k_policy.cc",
+    "src/process_mitigations_win32k_policy.h",
     "src/process_thread_dispatcher.cc",
     "src/process_thread_dispatcher.h",
     "src/process_thread_interception.cc",