Have net-export's save dialog default to last used save directory

chrome/browser/ui/webui/net_export_ui.cc

Index: chrome/browser/ui/webui/net_export_ui.cc
diff --git a/chrome/browser/ui/webui/net_export_ui.cc b/chrome/browser/ui/webui/net_export_ui.cc
index 313d5ee53e953b57bcb3dd52e8718a26dc8e5ef5..dc9b56286e7cb50886b42b29bad8c9a964b5c944 100644
--- a/chrome/browser/ui/webui/net_export_ui.cc
+++ b/chrome/browser/ui/webui/net_export_ui.cc
@@ -9,11 +9,13 @@
 
 #include "base/bind.h"
 #include "base/files/file_util.h"
+#include "base/lazy_instance.h"
 #include "base/macros.h"
 #include "base/strings/string_util.h"
 #include "base/strings/utf_string_conversions.h"
 #include "base/values.h"
 #include "chrome/browser/browser_process.h"
+#include "chrome/browser/download/download_prefs.h"
 #include "chrome/browser/platform_util.h"
 #include "chrome/browser/profiles/profile.h"
 #include "chrome/browser/ui/chrome_select_file_policy.h"
@@ -41,6 +43,9 @@ using content::WebUIMessageHandler;
 
 namespace {
 
+base::LazyInstance<base::FilePath>::Leaky
+    last_save_dir = LAZY_INSTANCE_INITIALIZER;

[mmenke, 2016/11/17 18:50:50]

Comment that this may only be accessed on the UI thread?  And also add 
DCHECK_CURRENTLY_ON(content::BrowserThread::UI); at the start of both methods
that access it?

Unsafe to read/write to it on multiple threads.

[wangyix, 2016/11/17 19:46:15]

Done.

+
 content::WebUIDataSource* CreateNetExportHTMLSource() {
   content::WebUIDataSource* source =
       content::WebUIDataSource::Create(chrome::kChromeUINetExportHost);
@@ -194,10 +199,13 @@ void NetExportMessageHandler::OnStartNetLog(const base::ListValue* list) {
   if (UsingMobileUI()) {
     StartNetLog();
   } else {
-    base::FilePath home_dir = base::GetHomeDir();
-    base::FilePath default_path =
-        home_dir.Append(FILE_PATH_LITERAL("chrome-net-export-log.json"));
-    ShowSelectFileDialog(default_path);
+    base::FilePath initial_dir = last_save_dir.Pointer()->empty() ?
+        DownloadPrefs::FromBrowserContext(
+            web_ui()->GetWebContents()->GetBrowserContext())->DownloadPath() :
+        *last_save_dir.Pointer();
+    base::FilePath initial_path =
+        initial_dir.Append(FILE_PATH_LITERAL("chrome-net-export-log.json"));
+    ShowSelectFileDialog(initial_path);
   }
 }
 
@@ -335,6 +343,7 @@ void NetExportMessageHandler::FileSelected(const base::FilePath& path,
   DCHECK(select_file_dialog_);
   select_file_dialog_ = nullptr;
 
+  *last_save_dir.Pointer() = path.DirName();
   BrowserThread::PostTaskAndReply(
       BrowserThread::FILE_USER_BLOCKING, FROM_HERE,
       base::Bind(&net_log::NetLogFileWriter::SetUpNetExportLogPath,