Add clear_fast_ink_buffer

This can be used to ensure that the fast ink buffer is cleared for
platforms that do not zero out new buffers.

BUG=b:168735625
TEST=build+deploy and inspect /etc/ui_use_flags.txt

Change-Id: I75e0af02b9109b4f953964f8c5d7c14e3847267f
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/overlays/chromiumos-overlay/+/2438030
Tested-by: Drew Davenport <ddavenport@chromium.org>
Reviewed-by: Benjamin Gordon <bmgordon@chromium.org>
Reviewed-by: Daniele Castagna <dcastagna@chromium.org>
Commit-Queue: Drew Davenport <ddavenport@chromium.org>
diff --git a/chromeos-base/libchromeos-use-flags/libchromeos-use-flags-9999.ebuild b/chromeos-base/libchromeos-use-flags/libchromeos-use-flags-9999.ebuild
index 85155a9..af386e3 100644
--- a/chromeos-base/libchromeos-use-flags/libchromeos-use-flags-9999.ebuild
+++ b/chromeos-base/libchromeos-use-flags/libchromeos-use-flags-9999.ebuild
@@ -37,6 +37,7 @@
 	borealis_host
 	cfm_enabled_device
 	cheets
+	clear_fast_ink_buffer
 	compupdates
 	diagnostics
 	disable_background_blur