Add a field trial to enable out-of-process V8 pac.

At the same time, add a "disable" flag as an escape hatch for when
things go bad.

BUG=467832

Review URL: https://codereview.chromium.org/1124083004

Cr-Commit-Position: refs/heads/master@{#330175}

chrome/browser/about_flags.cc

@@ -2341,7 +2341,8 @@ const Experiment kExperiments[] = {
     IDS_FLAGS_V8_PAC_MOJO_OUT_OF_PROCESS_NAME,
     IDS_FLAGS_V8_PAC_MOJO_OUT_OF_PROCESS_DESCRIPTION,
     kOsDesktop,
-    SINGLE_VALUE_TYPE(switches::kV8PacMojoOutOfProcess),
+    ENABLE_DISABLE_VALUE_TYPE(switches::kV8PacMojoOutOfProcess,
+                              switches::kDisableOutOfProcessPac)
   },
 #if defined(ENABLE_MEDIA_ROUTER)
   {

chrome/browser/net/proxy_service_factory.cc

@@ -7,6 +7,7 @@
 #include <string>
 
 #include "base/command_line.h"
+#include "base/metrics/field_trial.h"
 #include "base/strings/string_number_conversions.h"
 #include "base/threading/thread.h"
 #include "chrome/browser/browser_process.h"
@@ -38,6 +39,23 @@
 
 using content::BrowserThread;
 
+namespace {
+
+#if !defined(OS_ANDROID)
+bool EnableOutOfProcessV8Pac(const base::CommandLine& command_line) {
+  const std::string group_name =
+      base::FieldTrialList::FindFullName("OutOfProcessPac");
+
+  if (command_line.HasSwitch(switches::kDisableOutOfProcessPac))
+    return false;
+  if (command_line.HasSwitch(switches::kV8PacMojoOutOfProcess))
+    return true;
+  return group_name == "Enabled";
+}
+#endif  // !defined(OS_ANDROID)
+
+}  // namespace
+
 // static
 net::ProxyConfigService* ProxyServiceFactory::CreateProxyConfigService(
     PrefProxyConfigTracker* tracker) {
@@ -144,14 +162,16 @@ net::ProxyService* ProxyServiceFactory::CreateProxyService(
 #endif
 
 #if !defined(OS_ANDROID)
-    if (command_line.HasSwitch(switches::kV8PacMojoOutOfProcess)) {
-      proxy_service = net::CreateProxyServiceUsingMojoFactory(
-          UtilityProcessMojoProxyResolverFactory::GetInstance(),
+    // In-process Mojo PAC can only be set on the command line, so its presence
+    // should override other options.
+    if (command_line.HasSwitch(switches::kV8PacMojoInProcess)) {
+      proxy_service = net::CreateProxyServiceUsingMojoInProcess(
           proxy_config_service, new net::ProxyScriptFetcherImpl(context),
           dhcp_proxy_script_fetcher, context->host_resolver(), net_log,
           network_delegate);
-    } else if (command_line.HasSwitch(switches::kV8PacMojoInProcess)) {
-      proxy_service = net::CreateProxyServiceUsingMojoInProcess(
+    } else if (EnableOutOfProcessV8Pac(command_line)) {
+      proxy_service = net::CreateProxyServiceUsingMojoFactory(
+          UtilityProcessMojoProxyResolverFactory::GetInstance(),
           proxy_config_service, new net::ProxyScriptFetcherImpl(context),
           dhcp_proxy_script_fetcher, context->host_resolver(), net_log,
           network_delegate);

chrome/common/chrome_switches.cc

@@ -291,6 +291,9 @@ const char kDisableOfflineAutoReload[]       = "disable-offline-auto-reload";
 const char kDisableOfflineAutoReloadVisibleOnly[] =
     "disable-offline-auto-reload-visible-only";
 
+// Disable out-of-process V8 proxy resolver.
+const char kDisableOutOfProcessPac[] = "disable-out-of-process-pac";
+
 // Disable the setting to prompt the user for their OS account password before
 // revealing plaintext passwords in the password manager.
 const char kDisablePasswordManagerReauthentication[] =

chrome/common/chrome_switches.h

@@ -86,6 +86,7 @@ extern const char kDisableNewOfflineErrorPage[];
 extern const char kDisableNTPOtherSessionsMenu[];
 extern const char kDisableOfflineAutoReload[];
 extern const char kDisableOfflineAutoReloadVisibleOnly[];
+extern const char kDisableOutOfProcessPac[];
 extern const char kDisablePasswordManagerReauthentication[];
 extern const char kDisablePdfMaterialUI[];
 extern const char kDisablePermissionsBubbles[];

tools/metrics/histograms/histograms.xml

@@ -56564,6 +56564,7 @@ To add a new entry, add it with any value and run test to compute valid value.
   <int value="-1835975804" label="disable-offline-auto-reload"/>
   <int value="-1833149810" label="enable-accessibility-tab-switcher"/>
   <int value="-1832575380" label="show-saved-copy"/>
+  <int value="-1832221649" label="disable-out-of-process-pac"/>
   <int value="-1821058653" label="enable-delay-agnostic-aec"/>
   <int value="-1767470652" label="out-of-process-pdf"/>
   <int value="-1751928267" label="disable-icon-ntp"/>