Change expand button to be focusable by default

Prior to this change, expand button inherits ToggleImageButton's default
focus behavior which is ACCESSIBLE_ONLY. This button should be focusable
by default as user can navigate/toggle the button using a keyboard.

Bug: 1001103
Change-Id: Ic698f3865ddcb8cc66b962292831c3d5a73fc29f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2510729Reviewed-by: Marijn Kruisselbrink <mek@chromium.org>
Commit-Queue: Wei Li <weili@chromium.org>
Cr-Commit-Position: refs/heads/master@{#823662}

chrome/browser/ui/views/native_file_system/native_file_system_usage_bubble_view.cc

@@ -39,6 +39,7 @@
 #include "ui/views/controls/scroll_view.h"
 #include "ui/views/controls/table/table_view.h"
 #include "ui/views/layout/box_layout.h"
+#include "ui/views/style/platform_style.h"
 
 namespace {
 
@@ -167,6 +168,9 @@ class CollapsibleListView : public views::View {
         l10n_util::GetStringUTF16(IDS_NATIVE_FILE_SYSTEM_USAGE_EXPAND));
     button->SetToggledTooltipText(
         l10n_util::GetStringUTF16(IDS_NATIVE_FILE_SYSTEM_USAGE_COLLAPSE));
+    // TODO(weili): remove this once ToggleImageButton has the default platform
+    // focus behavior.
+    button->SetFocusBehavior(views::PlatformStyle::DefaultFocusBehavior());
     expand_collapse_button_ = label_container->AddChildView(std::move(button));
     if (model->RowCount() < 3)
       expand_collapse_button_->SetVisible(false);