Add the media migration main flag.

Bug: 340934031
Flag: EXEMPT - adding a flag
Test: N/A
Change-Id: I71da43c06bdbbfbe8578ffcd696aa44d93fe516e

Author: Carlos Martinez Romero

libs/gui/libgui_flags.aconfig

@@ -142,3 +142,11 @@ flag {
     purpose: PURPOSE_BUGFIX
   }
 } # bq_gl_fence_cleanup
+
+flag {
+  name: "wb_media_migration"
+  namespace: "core_graphics"
+  description: "Main flag for the warren buffers media migration."
+  bug: "340934031"
+  is_fixed_read_only: true
+} # wb_media_migration