Move kInvalidDisplayID to display_constants.h.

ui/display/types/display_constants.h

Index: ui/display/types/display_constants.h
diff --git a/ui/display/types/display_constants.h b/ui/display/types/display_constants.h
index b24bab7e6373dd81f3f9556fa9b2f85e1df75d51..73767579fd5fe042f96c7692d68802bb134c250a 100644
--- a/ui/display/types/display_constants.h
+++ b/ui/display/types/display_constants.h
@@ -5,6 +5,16 @@
 #ifndef UI_DISPLAY_TYPES_DISPLAY_CONSTANTS_H_
 #define UI_DISPLAY_TYPES_DISPLAY_CONSTANTS_H_
 
+#include <stdint.h>
+
+namespace display {
+
+// Display id that represents an invalid display. Often used as a default value
+// before display ids are known.

[Daniel Erat, 2016/11/21 17:33:57]

nit: s/id/ID/g in this comment

[kylechar, 2016/11/21 17:51:18]

Done.

+constexpr int64_t kInvalidDisplayID = -1;

[Daniel Erat, 2016/11/21 17:33:57]

it's a pretty big nit, but i think that "Id" is more common in constant names
than "ID":

% git grep 'const.*k[A-Z][a-zA-Z]*Id\b' | wc -l
3022
% git grep 'const.*k[A-Z][a-zA-Z]*ID\b' | wc -l
553

https://google.github.io/styleguide/cppguide.html#Function_Names touches on this
too: "Prefer to capitalize acronyms as single words (i.e. StartRpc(), not
StartRPC())." (a later sentence says that this also applies to constants.)

i realize that you're essentially just moving this out of a class, though, so
feel free to disregard this if it's a pain to update now. :-)

[kylechar, 2016/11/21 17:51:18]

I actually like kInvalidDisplayId better too. Changed.

+
+}  // namespace display
+
 namespace ui {
 
 // Used to describe the state of a multi-display configuration.