Revert "Remove QT feature flag"

This reverts commit bfc2cc87f3b7e24617d36ee98deed1f2209c0de3.

Reason for revert: Scale factor not updated on change (https://crbug.com/1439149)

Original change's description:
> Remove QT feature flag
>
> The feature has received launch approval.
>
> R=sky
>
> Change-Id: Icc9bff102bcdfabd6ce548fa306efa1183bb2246
> Bug: 1317782
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4408764
> Reviewed-by: Scott Violet <[email protected]>
> Commit-Queue: Thomas Anderson <[email protected]>
> Auto-Submit: Thomas Anderson <[email protected]>
> Cr-Commit-Position: refs/heads/main@{#1128358}

Bug: 1317782, 1439149
Change-Id: I722f47816d0f88da31e58978cec78ea64135de0d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4549942
Commit-Queue: Thomas Anderson <[email protected]>
Commit-Queue: Lei Zhang <[email protected]>
Auto-Submit: Thomas Anderson <[email protected]>
Reviewed-by: Lei Zhang <[email protected]>
Cr-Commit-Position: refs/heads/main@{#1146766}
diff --git a/ui/linux/linux_ui_factory.h b/ui/linux/linux_ui_factory.h
index 7620b92..5d4f4f4 100644
--- a/ui/linux/linux_ui_factory.h
+++ b/ui/linux/linux_ui_factory.h
@@ -14,6 +14,9 @@
 class LinuxUiTheme;
 enum class SystemTheme : int;
 
+// TODO(https://crbug.com/1317782): Remove in M110.
+COMPONENT_EXPORT(LINUX_UI_FACTORY) BASE_DECLARE_FEATURE(kAllowQt);
+
 // Returns a LinuxUi for the default toolkit.  May create a LinuxUi instance if
 // one does not exist.  May return nullptr if no toolkits are available.
 COMPONENT_EXPORT(LINUX_UI_FACTORY)