Allow the user to cancel the capture region action, and show its active status in the tray.

ash/common/system/chromeos/palette/palette_ids.h

Index: ash/common/system/chromeos/palette/palette_ids.h
diff --git a/ash/common/system/chromeos/palette/palette_ids.h b/ash/common/system/chromeos/palette/palette_ids.h
index bc33f972325ad03c225ecb70399c108f04e8272d..31dfe1323b3a674d1e686637f8241cd5fb770250 100644
--- a/ash/common/system/chromeos/palette/palette_ids.h
+++ b/ash/common/system/chromeos/palette/palette_ids.h
@@ -14,7 +14,8 @@ namespace ash {
 // Palette tools are grouped into different categories. Each tool corresponds to
 // exactly one group, and at most one tool can be active per group. Actions are
 // actions the user wants to do, such as take a screenshot, and modes generally
-// change OS behavior, like showing a laser pointer instead of a cursor.
+// change OS behavior, like showing a laser pointer instead of a cursor. A mode
+// is active until the user completes the action or disables it.
 enum class PaletteGroup { ACTION, MODE };
 
 enum class PaletteToolId {