ozone: xkbcommon: Pre-compute masks when setting keymap

Currently, XkbKeyboardLayoutEngine::DifficultKeyboardCode performs
unnecessary re-computation of XKB masks for Shift and AltGr modifiers
when trying to decode key codes. This change proposes an optimization
consisting on pre-computing and caching those masks only once, when
the keymap is set, then saving some cpu cycles when decoding difficult
keysyms/chars.

R=spang@chromium.org

Bug: 896640
Change-Id: Ifad7511f94312777d057930caf37b5c96ffbbad8
Reviewed-on: https://chromium-review.googlesource.com/c/1489951Reviewed-by: Michael Spang <spang@chromium.org>
Commit-Queue: Nick Yamane <nickdiego@igalia.com>
Cr-Commit-Position: refs/heads/master@{#635760}

ui/events/ozone/layout/xkb/xkb_keyboard_layout_engine.cc

@@ -846,6 +846,8 @@ void XkbKeyboardLayoutEngine::SetKeymap(xkb_keymap* keymap) {
   // Update num lock mask.
   num_lock_mod_mask_ = num_lock_mask;
 #endif
+  shift_mod_mask_ = EventFlagsToXkbFlags(ui::EF_SHIFT_DOWN);
+  altgr_mod_mask_ = EventFlagsToXkbFlags(ui::EF_ALTGR_DOWN);
 }
 
 xkb_mod_mask_t XkbKeyboardLayoutEngine::EventFlagsToXkbFlags(
@@ -940,7 +942,7 @@ KeyboardCode XkbKeyboardLayoutEngine::DifficultKeyboardCode(
       if (multi->subtable[i].test_shift) {
         if (shift_character == kNonCharacter) {
           shift_character = XkbSubCharacter(xkb_keycode, xkb_flags, character,
-                                            ui::EF_SHIFT_DOWN);
+                                            shift_mod_mask_);
         }
         if (shift_character != multi->subtable[i].shift_character)
           continue;
@@ -948,7 +950,7 @@ KeyboardCode XkbKeyboardLayoutEngine::DifficultKeyboardCode(
       if (multi->subtable[i].test_altgr) {
         if (altgr_character == kNonCharacter) {
           altgr_character = XkbSubCharacter(xkb_keycode, xkb_flags, character,
-                                            ui::EF_ALTGR_DOWN);
+                                            altgr_mod_mask_);
         }
         if (altgr_character != multi->subtable[i].altgr_character)
           continue;
@@ -974,8 +976,7 @@ base::char16 XkbKeyboardLayoutEngine::XkbSubCharacter(
     xkb_keycode_t xkb_keycode,
     xkb_mod_mask_t base_flags,
     base::char16 base_character,
-    int ui_flags) const {
-  xkb_mod_mask_t flags = EventFlagsToXkbFlags(ui_flags);
+    xkb_mod_mask_t flags) const {
   if (flags == base_flags)
     return base_character;
   xkb_keysym_t keysym;

ui/events/ozone/layout/xkb/xkb_keyboard_layout_engine.h

@@ -68,6 +68,8 @@ class EVENTS_OZONE_LAYOUT_EXPORT XkbKeyboardLayoutEngine
   // Flag mask for num lock, which is always considered enabled in ChromeOS.
   xkb_mod_mask_t num_lock_mod_mask_ = 0;
 #endif
+  xkb_mod_mask_t shift_mod_mask_ = 0;
+  xkb_mod_mask_t altgr_mod_mask_ = 0;
 
   // Determines the Windows-based KeyboardCode (VKEY) for a character key,
   // accounting for non-US layouts. May return VKEY_UNKNOWN, in which case the
@@ -114,7 +116,7 @@ class EVENTS_OZONE_LAYOUT_EXPORT XkbKeyboardLayoutEngine
   base::char16 XkbSubCharacter(xkb_keycode_t xkb_keycode,
                                xkb_mod_mask_t base_flags,
                                base::char16 base_character,
-                               int ui_flags) const;
+                               xkb_mod_mask_t flags) const;
 
   // Callback when keymap file is loaded complete.
   void OnKeymapLoaded(const std::string& layout_name,