Introduce enableLegacyPatching flag for CSS patching

As part of the dark mode migration we are adding
`enableLegacyPatching:true` to all call sites of `appendStyle` (and
related methods). If a user passes `false` for that flag, no CSS colour
patching will occur.

This CL is therefore a no-op from a user's perspective as we pass `true`
on every call to maintain existing behaviour, but once we start
migrating we will turn the option to `false`. Long term, once all code
is migrated and the old patching is removed, we will remove this flag
entirely once again.

Bug: 1122511
Change-Id: I76b818e83b7d5ee0175e1548759373f53481e0ab
No-Presubmit: True
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/2516345
Commit-Queue: Jack Franklin <[email protected]>
Reviewed-by: Tim van der Lippe <[email protected]>
diff --git a/front_end/settings/FrameworkBlackboxSettingsTab.js b/front_end/settings/FrameworkBlackboxSettingsTab.js
index 5442a95..910d63e 100644
--- a/front_end/settings/FrameworkBlackboxSettingsTab.js
+++ b/front_end/settings/FrameworkBlackboxSettingsTab.js
@@ -79,7 +79,7 @@
 export class FrameworkBlackboxSettingsTab extends UI.Widget.VBox {
   constructor() {
     super(true);
-    this.registerRequiredCSS('settings/frameworkBlackboxSettingsTab.css');
+    this.registerRequiredCSS('settings/frameworkBlackboxSettingsTab.css', {enableLegacyPatching: true});
 
     const header = this.contentElement.createChild('div', 'header');
     header.textContent = i18nString(UIStrings.frameworkBlackboxing);
@@ -97,7 +97,7 @@
 
     this._list = new UI.ListWidget.ListWidget(this);
     this._list.element.classList.add('blackbox-list');
-    this._list.registerRequiredCSS('settings/frameworkBlackboxSettingsTab.css');
+    this._list.registerRequiredCSS('settings/frameworkBlackboxSettingsTab.css', {enableLegacyPatching: true});
 
     const placeholder = document.createElement('div');
     placeholder.classList.add('blackbox-list-empty');