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..1708bb45d359c388b8d99ad364babacfb67205db 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 +37,10 @@ using installer::InstallationState;
 const wchar_t GoogleUpdateSettings::kPoliciesKey[] =
     L"SOFTWARE\\Policies\\Google\\Update";
 const wchar_t GoogleUpdateSettings::kUpdatePolicyValue[] = L"UpdateDefault";
+const wchar_t GoogleUpdateSettings::kDownloadPreference[] =
+    L"DownloadPreference";
+const wchar_t GoogleUpdateSettings::kDownloadPreferenceCacheable[] =
+    L"cacheable";
 const wchar_t GoogleUpdateSettings::kUpdateOverrideValuePrefix[] = L"Update";
 const wchar_t GoogleUpdateSettings::kCheckPeriodOverrideMinutes[] =
     L"AutoUpdateCheckPeriodMinutes";
@@ -770,6 +775,21 @@ bool GoogleUpdateSettings::ReenableAutoupdates() {
   return true;
 }
 
+// Reads and sanitizes the value of
+// "HKLM\SOFTWARE\Policies\Google\Update\DownloadPreference". The current
+// implementation supports only "cacheable" as a valid policy option.

[waffles, 2016/01/21 01:56:08]

Do we want to consider an alternate sanitization policy (e.g. matches
`[a-z]{0-32}`), so that future alternatives do not require a Chrome build (just
an Omaha, Template, and Server change)?

[Sorin Jianu, 2016/01/21 02:53:55]

I've sanitized by hand since adding a dependency on re2 in the installer seemed
an unacceptable tradeoff.

+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(kDownloadPreference, &value) == ERROR_SUCCESS &&
+      value == GoogleUpdateSettings::kDownloadPreferenceCacheable) {
+    return value;
+  }
+  return base::string16();
+}
+
 void GoogleUpdateSettings::RecordChromeUpdatePolicyHistograms() {
   const bool is_multi_install = InstallUtil::IsMultiInstall(
       BrowserDistribution::GetDistribution(), IsSystemInstall());