Allow custom CanvasKit URL (flutter#19822)

Author: yjbanov

lib/web_ui/lib/src/engine/compositor/initialization.dart

@@ -17,7 +17,10 @@ const bool canvasKitForceCpuOnly =
 /// When CanvasKit pushes a new release to NPM, update this URL to reflect the
 /// most recent version. For example, if CanvasKit releases version 0.34.0 to
 /// NPM, update this URL to `https://unpkg.com/[email protected]/bin/`.
-const String canvasKitBaseUrl = 'https://unpkg.com/[email protected]/bin/';
+const String canvasKitBaseUrl = String.fromEnvironment(
+  'FLUTTER_WEB_CANVASKIT_URL',
+  defaultValue: 'https://unpkg.com/[email protected]/bin/',
+);
 
 /// Initialize CanvasKit.
 ///