Initial implementation for sharing field trial state (win)

content/browser/browser_child_process_host_impl.cc

Index: content/browser/browser_child_process_host_impl.cc
diff --git a/content/browser/browser_child_process_host_impl.cc b/content/browser/browser_child_process_host_impl.cc
index e3eacd3ce01dead3132bee37df83760295395796..060ac6915589d80d8ca839febf79f27a1c1b59da 100644
--- a/content/browser/browser_child_process_host_impl.cc
+++ b/content/browser/browser_child_process_host_impl.cc
@@ -207,8 +207,13 @@ void BrowserChildProcessHostImpl::TerminateAll() {
 }
 
 // static
+#if defined(OS_WIN)
+void BrowserChildProcessHostImpl::CopyFeatureAndFieldTrialFlags(
+    base::CommandLine* cmd_line, base::SharedMemory* shared_memory) {
+#else
 void BrowserChildProcessHostImpl::CopyFeatureAndFieldTrialFlags(
     base::CommandLine* cmd_line) {
+#endif
   std::string enabled_features;
   std::string disabled_features;
   base::FeatureList::GetInstance()->GetFeatureOverrides(&enabled_features,
@@ -223,13 +228,29 @@ void BrowserChildProcessHostImpl::CopyFeatureAndFieldTrialFlags(
   std::string field_trial_states;
   base::FieldTrialList::AllStatesToString(&field_trial_states);
   if (!field_trial_states.empty()) {
+    // Use shared memory to pass field_trial_states if we're on Windows,
+    // otherwise fallback to the command line.
+#if defined(OS_WIN)
+    size_t length = field_trial_states.size() + 1;
+    shared_memory->CreateAndMapAnonymous(length);
+    memcpy(shared_memory->memory(), field_trial_states.c_str(), length);
+
+    // HANDLE is just typedef'd to void *
+    HANDLE handle = shared_memory->handle().GetHandle();
+    auto uintptr_handle = reinterpret_cast<std::uintptr_t>(handle);
+    std::string string_handle = std::to_string(uintptr_handle);
+    cmd_line->AppendSwitchASCII("field_trial_handle", string_handle);
+    cmd_line->AppendSwitchASCII("field_trial_length", std::to_string(length));
+#else
     cmd_line->AppendSwitchASCII(switches::kForceFieldTrials,
                                 field_trial_states);
+#endif
   }
 }
 
 void BrowserChildProcessHostImpl::Launch(
     SandboxedProcessLauncherDelegate* delegate,
+    base::SharedMemory* field_trial_state,
     base::CommandLine* cmd_line,
     bool terminate_on_shutdown) {
   DCHECK_CURRENTLY_ON(BrowserThread::IO);
@@ -259,6 +280,7 @@ void BrowserChildProcessHostImpl::Launch(
   notify_child_disconnected_ = true;
   child_process_.reset(new ChildProcessLauncher(
       delegate,
+      field_trial_state,
       cmd_line,
       data_.id,
       this,