Implement Windows GPO support for "dlpref" in component updater.

chrome/installer/util/google_update_settings.cc

Index: chrome/installer/util/google_update_settings.cc
diff --git a/chrome/installer/util/google_update_settings.cc b/chrome/installer/util/google_update_settings.cc
index e32e185d354c26607c7908f55f5c52ebb02a78c3..6ed2852de7797fcf4adba45b12e75797bde10438 100644
--- a/chrome/installer/util/google_update_settings.cc
+++ b/chrome/installer/util/google_update_settings.cc
@@ -8,6 +8,7 @@
 
 #include <algorithm>
 #include <limits>
+#include <vector>
 
 #include "base/command_line.h"
 #include "base/files/file_path.h"
@@ -36,6 +37,8 @@ using installer::InstallationState;
 const wchar_t GoogleUpdateSettings::kPoliciesKey[] =
     L"SOFTWARE\\Policies\\Google\\Update";
 const wchar_t GoogleUpdateSettings::kUpdatePolicyValue[] = L"UpdateDefault";
+const wchar_t GoogleUpdateSettings::kDownloadPreferencePolicyValue[] =
+    L"DownloadPreference";
 const wchar_t GoogleUpdateSettings::kUpdateOverrideValuePrefix[] = L"Update";
 const wchar_t GoogleUpdateSettings::kCheckPeriodOverrideMinutes[] =
     L"AutoUpdateCheckPeriodMinutes";
@@ -770,6 +773,30 @@ bool GoogleUpdateSettings::ReenableAutoupdates() {
   return true;
 }
 
+// Reads and sanitizes the value of
+// "HKLM\SOFTWARE\Policies\Google\Update\DownloadPreference". A valid
+// group policy option must be a single alpha numeric word of up to 32
+// characters.
+base::string16 GoogleUpdateSettings::GetDownloadPreference() {
+  RegKey policy_key;
+  base::string16 value;
+  if (policy_key.Open(HKEY_LOCAL_MACHINE, kPoliciesKey, KEY_QUERY_VALUE) ==
+          ERROR_SUCCESS &&
+      policy_key.ReadValue(kDownloadPreferencePolicyValue, &value) ==
+          ERROR_SUCCESS) {
+    // Validates that |value| matches `[a-zA-z]{0-32}`.
+    const size_t kMaxValueLength = 32;
+    if (value.size() > kMaxValueLength)
+      return base::string16();
+    for (auto ch : value) {
+      if (!base::IsAsciiAlpha(ch))
+        return base::string16();
+    }
+    return value;
+  }
+  return base::string16();
+}
+
 void GoogleUpdateSettings::RecordChromeUpdatePolicyHistograms() {
   const bool is_multi_install = InstallUtil::IsMultiInstall(
       BrowserDistribution::GetDistribution(), IsSystemInstall());