GN: Force normal subprocess task priority on Windows

In some cases, at least when starting as a scheduled task, Windows
subprocesses are started with a very low priority. Therefore, force
normal process priority when starting a subprocess.

R=dpranke@chromium.org

Change-Id: Ib9e8f9bb1f6d06cffeeab3694508df9e755f8a19
Reviewed-on: https://chromium-review.googlesource.com/941229Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Yngve Pettersen <yngve@vivaldi.com>
Cr-Commit-Position: refs/heads/master@{#540349}

tools/gn/exec_process.cc

@@ -93,7 +93,7 @@ bool ExecProcess(const base::CommandLine& cmdline,
                      &cmdline_str[0],
                      nullptr, nullptr,
                      TRUE,  // Handles are inherited.
-                     0, nullptr,
+                     NORMAL_PRIORITY_CLASS, nullptr,
                      startup_dir.value().c_str(),
                      &start_info, &temp_process_info)) {
     return false;