Revert of Fix sandbox target in Windows GN build and enable it. (patchset #1...

Revert of Fix sandbox target in Windows GN build and enable it. (patchset #1 id:1 of https://codereview.chromium.org/804993003/)

Reason for revert:
breaks compile step on the win8 gn bot: http://build.chromium.org/p/chromium.win/builders/Win8%20GN/builds/2037

Original issue's description:
> Fix sandbox target in Windows GN build and enable it.
>
> Committed: https://crrev.com/fdb4e8d624f1bfe1af5ccfebfa3faa0072caa71d
> Cr-Commit-Position: refs/heads/master@{#308471}

TBR=cpu@chromium.org,vchigrin@yandex-team.ru
NOTREECHECKS=true
NOTRY=true

Review URL: https://codereview.chromium.org/810623002

Cr-Commit-Position: refs/heads/master@{#308478}

BUILD.gn

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

sandbox/win/BUILD.gn

@@ -65,12 +65,6 @@ 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",