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

chrome/installer/setup/setup_util.h

Index: chrome/installer/setup/setup_util.h
diff --git a/chrome/installer/setup/setup_util.h b/chrome/installer/setup/setup_util.h
index cfd993f0d2509fb31c52ed21859f79b097c8ebf8..780c8832f921e6112b69bb91274122fe219d6298 100644
--- a/chrome/installer/setup/setup_util.h
+++ b/chrome/installer/setup/setup_util.h
@@ -81,6 +81,11 @@ bool WillProductBePresentAfterSetup(
     const installer::InstallationState& machine_state,
     BrowserDistribution::Type type);
 
+// Drops the process down to background processing mode on supported OSes if it
+// was launched below the normal process priority. Returns true when background
+// procesing mode is entered.
+bool AdjustProcessPriority();
+
 // This class will enable the privilege defined by |privilege_name| on the
 // current process' token. The privilege will be disabled upon the
 // ScopedTokenPrivilege's destruction (unless it was already enabled when the