Capture output from sandboxed child processes on the bots

This should allow stack traces written to stderr by base::debug::StackTrace to
show up in swarming output.

R=csharp

Bug: 871924
Change-Id: I9016603330fc2108837852a43fd17b7a04f2dffb
Reviewed-on: https://chromium-review.googlesource.com/1244552
Commit-Queue: Joe Mason <joenotcharles@chromium.org>
Reviewed-by: Chris Sharp <csharp@chromium.org>
Cr-Commit-Position: refs/heads/master@{#594433}

chrome/chrome_cleaner/ipc/sandbox.cc

@@ -114,7 +114,13 @@ scoped_refptr<sandbox::TargetPolicy> GetSandboxPolicy(
     LOG_IF(ERROR, sandbox_result != sandbox::SBOX_ALL_OK)
         << "Failed to give the target process access to the product directory";
   }
-#endif
+
+  // Also let it write to stdout and stderr. (If they point to the Windows
+  // console, these calls will silently fail. But this will let output from the
+  // child be captured on the bots or in the msys terminal.)
+  policy->SetStdoutHandle(::GetStdHandle(STD_OUTPUT_HANDLE));
+  policy->SetStderrHandle(::GetStdHandle(STD_ERROR_HANDLE));
+#endif  // CHROME_CLEANER_OFFICIAL_BUILD
 
   policy->SetLockdownDefaultDacl();