Fix keyboard-activated context menus

In Ash, there is logic to close menus then asynchronisly perform an accelerator command. However, the accelerator that opens the context menu itself can and does trigger the close path depending on how a person executes the command.

On Chrome OS, the shortcut to open a context menu is Search+Shift+Volume Up. Any of these keys should leave the menu as is otherwise, the context menu is effectively a no-op when triggered from the keyboard.

TEST=on device, hit search; verify apps list opens. Press Search+Shift+Volume up; verify context menu opens. Press Search+m with ChromeVox on, verify context menu opens.
BUG=621331,616130

Review-Url: https://codereview.chromium.org/2434483003
Cr-Commit-Position: refs/heads/master@{#427178}

ash/common/accelerators/accelerator_table.cc

@@ -528,6 +528,7 @@ const AcceleratorAction kActionsKeepingMenuOpen[] = {
     DISABLE_CAPS_LOCK,
     KEYBOARD_BRIGHTNESS_DOWN,
     KEYBOARD_BRIGHTNESS_UP,
+    TOGGLE_APP_LIST,
     TOGGLE_CAPS_LOCK,
     TOGGLE_HIGH_CONTRAST,
     TOGGLE_SPOKEN_FEEDBACK,