gn win: re-add event_recorder_stubs.cc, mistakenly removed before

(This matches the gyp build)

R=brettw@chromium.org
TBR=brettw@chromium.org
BUG=354261

Review URL: https://codereview.chromium.org/467413002

Cr-Commit-Position: refs/heads/master@{#289657}
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@289657 0039d316-1c4b-4281-b951-d872f2087c98

base/BUILD.gn

@@ -173,6 +173,7 @@ component("base") {
     "environment.cc",
     "environment.h",
     "event_recorder.h",
+    "event_recorder_stubs.cc",
     "event_recorder_win.cc",
     "file_descriptor_posix.h",
     "file_version_info.h",