x11: Fix regression in Neo layout 4th layer keys decoding

Map EF_MOD3_DOWN to GDK_MOD_MASK when converting ui::KeyEvent into
GdkEventKey before injecting key events into GtkIM. Mapping it to
GDK_META_MASK is wrong and breaks Neo layout Layer 4 keys decoding.

Bug: 1025376
Change-Id: I34afb73e006278e78f64b9b084732ee1176f70ba
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1917052Reviewed-by: Thomas Anderson <thomasanderson@chromium.org>
Commit-Queue: Nick Yamane <nickdiego@igalia.com>
Cr-Commit-Position: refs/heads/master@{#715962}

chrome/browser/ui/libgtkui/gtk_util.cc

@@ -95,7 +95,7 @@ GdkModifierType ExtractGdkEventStateFromKeyEvent(
       {ui::EF_CONTROL_DOWN, GDK_CONTROL_MASK},
       {ui::EF_ALT_DOWN, GDK_MOD1_MASK},
       {ui::EF_NUM_LOCK_ON, GDK_MOD2_MASK},
-      {ui::EF_MOD3_DOWN, GDK_META_MASK},
+      {ui::EF_MOD3_DOWN, GDK_MOD3_MASK},
       {ui::EF_COMMAND_DOWN, GDK_MOD4_MASK},
       {ui::EF_ALTGR_DOWN, GDK_MOD5_MASK},
       {ui::EF_LEFT_MOUSE_BUTTON, GDK_BUTTON1_MASK},