Prevent Shift+Tab from locking up the browser when in the app list.

This CL fixes an issue where Shift+Tab would lock up the whole chrome
browser process when in the app list. This has been fixed by explicitly
capturing the Shift+Tab event since there are no other focus targets
anyway.

BUG=438425

Review URL: https://codereview.chromium.org/791403004

Cr-Commit-Position: refs/heads/master@{#308308}

ui/app_list/views/search_box_view.cc

@@ -221,6 +221,13 @@ bool SearchBoxView::HandleKeyEvent(views::Textfield* sender,
   if (contents_view_ && contents_view_->visible())
     handled = contents_view_->OnKeyPressed(key_event);
 
+  // Prevent Shift+Tab from locking up the whole chrome browser process.
+  // Explicitly capturing the Shift+Tab event here compensates for a focus
+  // search issue. We get away with this because there are no other focus
+  // targets. See http://crbug.com/438425 for details.
+  if (key_event.key_code() == ui::VKEY_TAB && key_event.IsShiftDown())
+    handled = true;
+
   return handled;
 }