Lower the I/O priority of the installer when resonable.

chrome/installer/setup/setup_main.cc

Index: chrome/installer/setup/setup_main.cc
diff --git a/chrome/installer/setup/setup_main.cc b/chrome/installer/setup/setup_main.cc
index 70832b641e8244870b3d7785a7b632ca9a23f607..6056850129c244edce154f0e72f3b41b12061bcb 100644
--- a/chrome/installer/setup/setup_main.cc
+++ b/chrome/installer/setup/setup_main.cc
@@ -639,6 +639,11 @@ installer::InstallStatus InstallProductsHelper(
   const bool system_install = installer_state.system_install();
   installer::InstallStatus install_status = installer::UNKNOWN_STATUS;
 
+  // Drop to background processing mode if the process was started below the
+  // normal process priority class.
+  VLOG_IF(1, installer::AdjustProcessPriority())

[cpu_(ooo_6.6-7.5), 2013/06/03 19:43:02]

I am always scared of calling actuall code inside logging macros. Eventually
somebody is going to create a configuration where this is a no op.

[grt (UTC plus 2), 2013/06/03 20:13:18]

Done.

+      << "Entered background processing mode.";
+
   // For install the default location for chrome.packed.7z is in current
   // folder, so get that value first.
   base::FilePath archive(cmd_line.GetProgram().DirName().Append(