ash/chromeos: Add shift-alt-p shortcut to open palette.

ash/common/system/chromeos/palette/palette_tray.cc

Index: ash/common/system/chromeos/palette/palette_tray.cc
diff --git a/ash/common/system/chromeos/palette/palette_tray.cc b/ash/common/system/chromeos/palette/palette_tray.cc
index be92beccd1d7b8ca50c3fad62c4b15a9e2603116..3c1d9cc19b3a0a0e917bda7be8a7ebc0915df6af 100644
--- a/ash/common/system/chromeos/palette/palette_tray.cc
+++ b/ash/common/system/chromeos/palette/palette_tray.cc
@@ -171,6 +171,9 @@ bool PaletteTray::PerformAction(const ui::Event& event) {
 }
 
 bool PaletteTray::OpenBubble() {
+  if (bubble_)
+    return false;
+
   views::TrayBubbleView::InitParams init_params(
       views::TrayBubbleView::ANCHOR_TYPE_TRAY, GetAnchorAlignment(),
       kPaletteWidth, kPaletteWidth);
@@ -196,7 +199,6 @@ bool PaletteTray::OpenBubble() {
   bubble_.reset(new ash::TrayBubbleWrapper(this, bubble_view));
 
   SetDrawBackgroundAsActive(true);
-
   return true;
 }