[Windows Sandbox] Turn on MITIGATION_EXTENSION_POINT_DISABLE for child processes.

content/app/sandbox_helper_win.cc

Index: content/app/sandbox_helper_win.cc
diff --git a/content/app/sandbox_helper_win.cc b/content/app/sandbox_helper_win.cc
index 6e06e5c420fd1a7e8155294d9b7b2381998f3ecf..3791888e11ed91b93ea0744c82d44b6e1bde0132 100644
--- a/content/app/sandbox_helper_win.cc
+++ b/content/app/sandbox_helper_win.cc
@@ -19,6 +19,9 @@ void InitializeSandboxInfo(sandbox::SandboxInterfaceInfo* info) {
         sandbox::MITIGATION_DEP |
         sandbox::MITIGATION_DEP_NO_ATL_THUNK |
         sandbox::MITIGATION_HARDEN_TOKEN_IL_POLICY);
+    // Note: these mitigations are "post-startup".  Some mitigations that need
+    // to be enabled sooner (e.g. MITIGATION_EXTENSION_POINT_DISABLE) are done
+    // so in Chrome_ELF.
   }
 }