[NTP] Change upload image format and change description

Enable the .png image upload by default. Change the options drop down
menu to only have one option with hardcoded string "*.jpeg, *.jpg,
*png".

Screenshot:
https://screenshot.googleplex.com/1sd8JoBoM14.png
https://screenshot.googleplex.com/5mnsxOtRxJD.png

Bug: 864303
Change-Id: I3b041d0f4e08898fa0c6998742a8ce04065ab5ac
Reviewed-on: https://chromium-review.googlesource.com/1166027
Commit-Queue: Weilun Shi <sweilun@chromium.org>
Reviewed-by: Kristi Park <kristipark@chromium.org>
Cr-Commit-Position: refs/heads/master@{#581360}

chrome/app/generated_resources.grd

@@ -4705,6 +4705,9 @@ Keep your key file in a safe place. You will need it to create new versions of y
       <message name="IDS_NTP_CUSTOM_LINKS_EDIT_SHORTCUT" desc="The title for custom link edit shortcut dialog. (On the New Tab Page)">
         Edit shortcut
       </message>
+      <message name="IDS_UPLOAD_IMAGE_FORMAT" desc="The permitted upload image format for the custom background. (On the New Tab Page)">
+        *.jpeg, *.jpg, *.png
+      </message>
       <message name="IDS_NTP_CUSTOM_LINKS_NAME" desc="The title for custom link name input field on the edit shortcut dialog. (On the New Tab Page)">
         Name
       </message>

chrome/browser/ui/search/search_tab_helper.cc

@@ -440,10 +440,12 @@ void SearchTabHelper::OnSelectLocalBackgroundImage() {
 
   ui::SelectFileDialog::FileTypeInfo file_types;
   file_types.allowed_paths = ui::SelectFileDialog::FileTypeInfo::NATIVE_PATH;
-  file_types.extensions.resize(2);
+  file_types.extensions.resize(1);
   file_types.extensions[0].push_back(FILE_PATH_LITERAL("jpg"));
   file_types.extensions[0].push_back(FILE_PATH_LITERAL("jpeg"));
-  file_types.extensions[1].push_back(FILE_PATH_LITERAL("png"));
+  file_types.extensions[0].push_back(FILE_PATH_LITERAL("png"));
+  file_types.extension_description_overrides.push_back(
+      l10n_util::GetStringUTF16(IDS_UPLOAD_IMAGE_FORMAT));
 
   select_file_dialog_->SelectFile(
       ui::SelectFileDialog::SELECT_OPEN_FILE, base::string16(), directory,