Fix PrintScreen key on Linux.

Despite the name, Linux/X11 systems map the PrintScreen
key to XK_Print rather than XK_KEY_3270_PrintScreen.
We map both to DOM 'PrintScreen' to match user expectations, and
the behaviour of other browsers, and Chrome on other platforms.

BUG=683097

Change-Id: If2d4f78ef191d51544f0eee64098a7348e60fa37
Reviewed-on: https://chromium-review.googlesource.com/921921
Commit-Queue: Wez <wez@chromium.org>
Reviewed-by: Wez <wez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#537512}

ui/events/keycodes/keyboard_code_conversion_xkb.cc

@@ -110,8 +110,10 @@ DomKey NonPrintableXKeySymToDomKey(xkb_keysym_t keysym) {
       return DomKey::END;
     case XKB_KEY_Select:
       return DomKey::SELECT;
+    // Treat Print/PrintScreen as PrintScreen https://crbug.com/683097.
     case XKB_KEY_Print:
-      return DomKey::PRINT;
+    case XKB_KEY_3270_PrintScreen:
+      return DomKey::PRINT_SCREEN;
     case XKB_KEY_Execute:
       return DomKey::EXECUTE;
     case XKB_KEY_Insert:
@@ -357,8 +359,6 @@ DomKey NonPrintableXKeySymToDomKey(xkb_keysym_t keysym) {
       return DomKey::EX_SEL;
     case XKB_KEY_3270_CursorSelect:
       return DomKey::CR_SEL;
-    case XKB_KEY_3270_PrintScreen:
-      return DomKey::PRINT_SCREEN;
     case XKB_KEY_ISO_Level3_Shift:
       return DomKey::ALT_GRAPH;
     case XKB_KEY_ISO_Level3_Latch: